Fix MT project-cache file access compatibility

[JanProvaznik]

Fixes #14824
Fixes #14825
Fixes #14826

Summary

• Preserve FileAccessData values across TaskHost deserialization without changing the existing payload format.
• Propagate ReportFileAccesses to TaskHost with packet-version compatibility, gate collection and replay, and keep reported accesses isolated per task execution context.
• Reject -mt together with -reportFileAccesses using actionable MSB1073; programmatic BuildManager callers receive the equivalent validation.
• Document why Detours attribution requires process-isolated worker nodes.

Compatibility

Project-cache lookup/materialization remains compatible with MT. Only Detours-based cache population is rejected because concurrent projects in one process cannot be attributed correctly. CLR2, CLR4, and negotiated .NET TaskHost packet formats retain compatible behavior.

Validation

• Repository build succeeds with zero warnings/errors.
• 14 focused regression tests pass.
• Command-line unit tests pass in Release: 482 net472 and 470 net11.0.
• Relevant engine regressions pass in Release.
• Final TestFX fork validation is green; trusted cache hit/miss validation is tracked in Experiment with trusted cache population and MT consumption microsoft/testfx#10834.

[JanProvaznik]

/review