Simplify build-trace gate; remove dynamic subcommand probe

[eseidel]

Two changes, stacked:

1. fix(cli): gate build tracing off for unknown Flutter versions

prepareBuildTrace used flutterVersion ?? minVersion to handle revisions that aren't on any flutter_release/* branch (dev pins). That fallback is wrong for a feature-enable check: a Shorebird-fork dev revision predating the tracing PR would be treated as new enough, get --shorebird-trace forwarded to flutter, and fail argparse hard.

Flip FlutterSupportConstraint.isSatisfiedBy to accept Version? and require either a real version ≥ the floor or an allowlisted revision to satisfy it. Unknown + non-allowlisted now leaves the trace session off. Added c2c56ab2d5483bdf86152725342f55ca6faed946 (the current pin) to the allowlist — previously it was carried implicitly by #3708's runtime probe, which this PR also removes.

2. Revert "fix: only pass --shorebird-trace to build commands that support it (#3708)"

#3708 probed each flutter build <cmd> -h at runtime to decide whether to pass --shorebird-trace. That was a belt-and-suspenders guard around two real bugs:

1. The gate mis-classified unresolved versions (fixed by the previous commit).
2. flutter build aar / ios-framework / linux / macos / windows didn't register the option (fixed by the companion Extend --shorebird-trace to aar, ios-framework, and desktop builds flutter#136).

With both fixes in place, every subcommand this CLI invokes accepts the flag when the gate admits tracing, and every subcommand refuses it when the gate rejects tracing — so the per-subcommand runtime probe (plus its 8 per-command call-site detours and its help-output cache) is dead weight.

Landing order

This PR must land after shorebirdtech/flutter#136 is merged AND bin/internal/flutter.version is bumped to a revision that includes it. I reproduced a hard crash on shorebird release macos when this simplification is applied without the Flutter side (see Scenario C counter-factual below).

Test plan

• dart test packages/shorebird_cli/test/src/artifact_builder/artifact_builder_test.dart — 92 tests pass.

End-to-end on a fresh test project (/tmp/shorebird-trace-test/demo):

• Scenario A — Shorebird gate fix (alone). shorebird release android --dry-run --flutter-version be0b343f (a pre-trace Shorebird-fork dev hash, not on any flutter_release/* branch).
  • With both fixes: Command: flutter build appbundle --release --target-platform=android-arm,android-arm64,android-x64 — no --shorebird-trace.
  • Counter-factual, pre-fix baseline (git checkout 2d0f8919~1, same command): Could not find an option named "--shorebird-trace". Command: flutter build appbundle ... --shorebird-trace=/…/build-trace-android.json.
• Scenario B — current pin baseline. shorebird release android --dry-run (pin is allowlisted): builds AAB end-to-end, writes build/shorebird/debug/build-trace-android.json (17.8KB of valid Chrome Trace Event Format JSON).
• Scenario C — desktop. shorebird release macos --dry-run:
  • Counter-factual, this PR applied without the Flutter side: Could not find an option named "--shorebird-trace". Command: flutter build macos --release --shorebird-trace=…. This is the crash that motivates the landing-order note.
  • With the Flutter pin bumped to the companion PR's branch HEAD + that hash allowlisted: ✓ Built build/macos/Build/Products/Release/demo.app (48.6MB) / No issues detected.

[codecov]

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