Align HTML report filename path semantics with TRX

[Copilot]

--report-html-filename and --report-trx-filename looked like sibling options but accepted different path shapes. HTML rejected nested paths while TRX accepted relative paths, absolute paths, and placeholders.

• HTML filename validation

  • Allows bare filenames, relative paths, and absolute paths.
  • Rejects relative paths that escape the test results directory.
  • Keeps .html leaf extension validation.

• HTML report output

  • Resolves standard artifact placeholders: {pname}, {pid}, {asm}, {tfm}, {time}.
  • Creates missing output directories.
  • Preserves absolute-path override behavior, matching TRX.

• Help and tests

  • Updates help text to document the expanded contract.
  • Adds focused unit coverage for validation, placeholder resolution, sanitization, and directory handling.
  • Updates acceptance coverage for nested HTML report paths.

Example:

dotnet test --report-html --report-html-filename artifacts/run_{tfm}.html
dotnet test --report-html --report-html-filename /tmp/run.html