Emit compact end-of-run failure index for LLM/agent consumption

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## Summary

At the end of a `Microsoft.Testing.Platform` test run, emit a compact, sentinel-delimited footer that lists every failed test as a single line of `Test FQN | first-line error | file:line`. Default-on in LLM-detected environments only (see `LLMEnvironmentDetector`).

## Motivation

When a test run fails, an LLM agent currently has to re-read the entire terminal transcript to figure out what failed and where. The per-failure blocks are paragraph-style (message + expected/actual + stack), suitable for a human scanning visually but expensive in tokens when the agent only needs the structured summary to plan its next step.

A small machine-friendly footer at a stable position is the lowest-cost way to give the agent a "table of contents" of failures.

## Proposed shape

```
##[failures]
TestFx.UnitTests.FooTests.WhenBarThenBaz | Expected 42 but was 41 | src/Foo.cs:128
TestFx.UnitTests.FooTests.WhenBarThenQux | NullReferenceException: Object reference not set... | src/Foo.cs:142
##[/failures]
```

Properties:

- Sentinel-delimited (`##[failures]` ... `##[/failures]`) so an agent can extract the block with a 2-line regex without parsing the rest of the transcript.
- One line per failure. Fields separated by ` | `. Pipe-in-value escaped as `\|`.
- Test FQN comes from the test node display path.
- First-line error: first non-empty line of the failure message, truncated to ~200 chars (configurable).
- `file:line` extracted from the first user-code frame (after the filter from #8825 / related). Empty if not found.

## Behavior

- Default-on when `LLMEnvironmentDetector.IsLLMEnvironment()` returns `true`.
- Default-off otherwise, to keep the human terminal transcript stable.
- For v1 do not add a CLI option, to avoid `--help`/`--info` acceptance churn and the XLF round.
- If desired in v2, add `--failure-summary `.

## Implementation notes

- `TerminalTestReporter` already tracks per-assembly state and emits an `AppendTestRunSummary` at the end (see `TerminalTestReporter.Summary.cs`). The failure index should be appended right after that summary block.
- Today `TerminalTestReporter.TestCompleted` renders failed details and discards them. To build the index, capture a bounded list of `(displayName, firstErrorLine, firstUserCodeFileLine)` per failed test as we render it.
- Cap the captured list at e.g. 5,000 entries to avoid OOM in giant runs; if exceeded, emit `## ...truncated, N more failures...`.
- Verify behavior under multi-assembly runs (one footer per assembly vs one global footer at the end — global at the end is preferable for agents).

## Risks

- Adding any new lines to the terminal transcript can break existing scrapers. Mitigation: LLM-mode-gated default.
- Capturing failure state grows reporter memory. Mitigation: cap + truncate.

## Acceptance criteria

- In LLM mode, after a failed run the transcript contains exactly one `##[failures]`/`##[/failures]` block listing all failures.
- In human mode (no LLM env vars), output is identical to today.
- Tests cover: zero failures (no block), single failure, many failures, multi-line error message, missing file path.

## References

- Umbrella: #8824
- Files: `src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal/TerminalTestReporter.*.cs`
- Inspiration: GitHub Actions log grouping (`::group::`), Azure DevOps `##[error]`/`##vso[task.logissue]` lines.

Start with TerminalTestReporter.TestCompleted and TerminalTestReporter.Summary.cs, then inspect LLMEnvironmentDetector and the files under src/Platform/Microsoft.Testing.Platform/OutputDevice/Terminal. Verify the footer is emitted only in LLM mode, contains the required bounded failure entries, and remains absent in human mode. Add coverage for zero, single, many, multiline, missing-path, and multi-assembly failures.

Written by the indexing model from the issue text.