Replace TestKit XML documentation placeholders

[Aaronontheweb]

TestKit and related test-support APIs had 797 XML documentation placeholders in 310 blocks across 59 files. Document actor assertions, message reception, event filtering, virtual scheduling, timing bounds, test actor lifecycle, and adapter helpers, using the existing implementation and contracts. Preserve useful prose and leave executable source, signatures, attributes, nullable directives, ordinary comments, tests, configuration, dependencies, and API approval files unchanged.

Luna authored the docs. Independent Sol review covered all 59 files and rechecked all corrections, including timeout/cancellation behavior, consumed versus retained messages, autopilots, event-filter assertions, virtual scheduler due buckets, and the actual no-message state used by Within. This PR is independent against dev.

Validation:

• Exact ownership scan: zero XML documentation placeholders. Roslyn comparison preserves executable tokens and all non-documentation trivia/directives.
• All eight affected Release project builds pass with warnings as errors. Compiler XML member IDs are unchanged; scoped DocFX metadata and rendered builds pass with zero warnings.
• Existing runtime coverage is carried from the source-equivalent Core documentation candidate: all 34 selected project suites have passing results (8,023 passed, 523 existing skips, zero failed tests). The original Incrementalist command failed on framework propagation to a source generator; Serialization V2 passed its 566 tests using the corrected command. CI runs required affected-project checks.
• Slopwatch with baseline disabled reports the same one existing error and six existing warnings, with zero introduced findings. No suppression or baseline edits. Actual IDE tooltip rendering was not exercised.

Combined local verification with all 13 batches also passes: 22 affected Release builds with warnings as errors, 33 API approvals, zero compiler-XML placeholders, and zero rendered-documentation warnings. Full metadata retains the same 576 baseline warnings; Slopwatch has no introduced findings. No PRs were merged by the agent.

The agent will not attempt to merge campaign PRs.