Enforce ConfigureAwait(false) across shipped code

[Vulthil]

Summary

• Adds src/.editorconfig enabling CA2007 as an error for every .cs file under src/ (tests/ and
  samples/ keep the repo-wide disabled default from the root .editorconfig).
• Mechanically adds .ConfigureAwait(false) to every await in src/, then hand-fixes the spots
  the automatic fixer mangled (see Verification).
• Adds one line to .github/copilot-instructions.md recording the policy.

NOTE: stacked on #353 — its base is the comment-policy-sweep branch so this diff is
the single ConfigureAwait commit. It gets retargeted to main after that PR merges.

Verification

• Scoping proved empirically both ways: src/ builds fail on an unconfigured await; a test
  project with real unconfigured awaits raises zero CA2007 from any test file.
• The formatter mis-handled every await using var x = expr.ConfigureAwait(false) where x's own
  members were used afterward, since wrapping the disposable changes its static type to
  ConfiguredAsyncDisposable. Hand-fixed ~15 locations by splitting into
  var x = expr; await using var _ = x.ConfigureAwait(false);, preserving both the real type and
  the configured disposal await. One ValueTask-coalescing expression surfaced new CA2012 +
  IDE0047 diagnostics under the transformation; rewritten properly rather than suppressed.
• Full solution build, both TFMs: clean, 0 warnings/errors.
• Full CI-parity suite (./eng/coverage.ps1), including the Aspire-hosted suite: 1582/1582
  passing on a clean run. An earlier run hit 4 failures under heavy parallel load; all 4 were
  independently confirmed as pre-existing timing-sensitive flakes unrelated to this change via
  isolated re-runs (each passed 100% standalone).
• PublicAPI*.txt / CompatibilitySuppressions.xml: byte-identical (ConfigureAwait is
  implementation-only).
• dotnet pack -c Release spot-run for two touched packages: both pack cleanly, no APICompat
  warnings.

Backport to v1.0: no — convention sweep ships with the next release.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.