Add structured assertion message infrastructure (RFC 012)

[Evangelink]

Summary

Introduces the core building blocks for structured assertion messages as described in RFC 012. This is the first in a series of PRs that will migrate all Assert.* methods to the new multi-line format.

Changes

New types

• EvidenceBlock — represents a labeled evidence section (e.g. expected: 42 / actual: 37) with automatic column-alignment of labels.
• StructuredAssertionMessage — builder that composes the full multi-line message: assertion prefix, summary, optional user message, evidence blocks, and call-site expression.
• AssertionValueRenderer — renders values for evidence blocks following the RFC's value rendering rules (strings quoted, nulls unquoted, collections in [...] notation, control characters escaped).

Updated types

• AssertFailedException — added ExpectedText and ActualText properties to carry structured expected/actual values for IDE integration.
• Assert — added ReportAssertFailed and ThrowAssertFailed overloads that accept StructuredAssertionMessage.

Tests

• StructuredAssertionMessageTests — verifies message layout, blank-line separators, multi-block formatting, and edge cases.
• EvidenceBlockTests — verifies label alignment, multi-line values, and single-line formatting.
• AssertionValueRendererTests — verifies rendering of nulls, strings, numbers, booleans, collections, and objects.
• AssertFailedExceptionTests — verifies the new ExpectedText/ActualText properties.

Public API

New public API entries added to PublicAPI.Unshipped.txt.

Follow-up PRs

1. This PR — Infrastructure
2. Boolean & Null assertions (IsTrue, IsFalse, IsNull, IsNotNull)
3. Equality assertions (AreEqual, AreNotEqual)
4. Reference & Type assertions (AreSame, AreNotSame, IsInstanceOfType, etc.)
5. Exception assertions (Throws, ThrowsExactly, ThrowsAsync, etc.)
6. String assertions (Contains, StartsWith, EndsWith, MatchesRegex, etc.)
7. Collection assertions
8. Comparison + Fail / Inconclusive

[Copilot]

Pull request overview

This PR introduces the initial infrastructure to support RFC 012 structured assertion failure messages in the MSTest TestFramework, including message composition, evidence block formatting, and value rendering, along with plumbing for carrying structured expected/actual values on AssertFailedException.

Changes:

• Added internal structured message builder and evidence formatting types (StructuredAssertionMessage, EvidenceBlock, EvidenceLine) plus a value renderer (AssertionValueRenderer).
• Extended AssertFailedException with ExpectedText/ActualText and added structured-message overloads in Assert to populate them.
• Added unit tests covering message layout, evidence alignment, and basic value rendering.

Show a summary per file

File	Description
test/UnitTests/TestFramework.UnitTests/Assertions/StructuredAssertionMessageTests.cs	New tests validating multi-line structured message layout rules.
test/UnitTests/TestFramework.UnitTests/Assertions/EvidenceBlockTests.cs	New tests validating evidence label alignment and formatting.
test/UnitTests/TestFramework.UnitTests/Assertions/AssertionValueRendererTests.cs	New tests validating value rendering rules (null/strings/bools/chars/collections/etc.).
test/UnitTests/TestFramework.UnitTests/Assertions/AssertFailedExceptionTests.cs	New tests validating the new ExpectedText/ActualText properties.
src/TestFramework/TestFramework/PublicAPI/PublicAPI.Unshipped.txt	Declares the new public API getters for ExpectedText/ActualText.
src/TestFramework/TestFramework/Exceptions/AssertFailedException.cs	Adds ExpectedText/ActualText properties to the exception type.
src/TestFramework/TestFramework/Assertions/StructuredAssertionMessage.cs	Implements the structured message builder and formatter.
src/TestFramework/TestFramework/Assertions/EvidenceLine.cs	Adds a line-level evidence representation (label/value).
src/TestFramework/TestFramework/Assertions/EvidenceBlock.cs	Implements evidence block formatting with per-block label alignment.
src/TestFramework/TestFramework/Assertions/AssertionValueRenderer.cs	Implements RFC-aligned rendering for strings/control chars/collections/etc.
src/TestFramework/TestFramework/Assertions/Assert.cs	Adds structured-message overloads and wires them to AssertFailedException.

Copilot's findings

• Files reviewed: 11/11 changed files
• Comments generated: 4

[Copilot]

Copilot's findings

Comments suppressed due to low confidence (1)

src/TestFramework/TestFramework/Assertions/EvidenceBlock.cs:63

• RFC 012 specifies that multi-line evidence values should be formatted with the label on its own line (e.g. expected:) and the value starting on the next line, flush-left. EvidenceBlock.Format currently always renders label + ' ' + value, which will produce malformed output when value contains newlines and doesn't meet the RFC/PR description ("multi-line values"). Please update the formatting logic (and add a test) to handle multi-line values explicitly.

    /// <summary>
    /// Formats the evidence block as aligned label: value lines.
    /// Labels are right-padded so all values start at the same column.
    /// </summary>
    internal string Format()
    {
        if (_lines.Count == 0)
        {
            return string.Empty;
        }

        int maxLabelLength = 0;
        foreach (EvidenceLine line in _lines)
        {
            if (line.Label.Length > maxLabelLength)
            {
                maxLabelLength = line.Label.Length;
            }
        }

        StringBuilder sb = new();
        for (int i = 0; i < _lines.Count; i++)
        {
            if (i > 0)
            {
                sb.Append(Environment.NewLine);
            }

            EvidenceLine line = _lines[i];

            // Pad label (which includes trailing colon) to align values, then append a space and value
            sb.Append(line.Label.PadRight(maxLabelLength));
            sb.Append(' ');
            sb.Append(line.Value);
        }

• Files reviewed: 11/11 changed files
• Comments generated: 4