Migrate platform channels to Pigeon 29

[fotiDim]

Alternative to #347: the same Pigeon 29 regeneration, but preserving the existing callback-style native code via @asyncCallback instead of migrating the plugins to native async/suspend.

Approach

Pigeon ≥ 28 changes the generated channel shape by default:

• @HostApi methods annotated @async → suspend (Kotlin) / async throws (Swift)
• @FlutterApi methods → async by default

Annotating the async HostApi methods and all FlutterApi methods with @asyncCallback keeps the generated native signatures callback-based, so the hand-written plugins need no changes and no coroutines dependency is required. C++ was already callback-based and is unaffected either way.

Changes

• pigeon/universal_ble.dart: @async → @asyncCallback (19 HostApi methods) and @asyncCallback added to all 15 FlutterApi methods.
• Regenerated all five outputs to v29.0.10 (lib/src/universal_ble.g.dart, UniversalBle.g.kt, UniversalBle.g.swift, windows/.../universal_ble.g.{h,cpp}); Dart file formatted.
• lib/src/universal_ble_pigeon/*.dart: the FlutterApi methods now return Future on the Dart side, so the two callback implementers were made async (bodies stay synchronous).
• CHANGELOG.md: 3.1.1 note.

No public Dart API change (the callback classes are not re-exported); still 3.1.1.

Differences from #347

• No native async rewrite: Kotlin stays callback-based (no suspendCancellableCoroutine, no kotlinx-coroutines), Swift stays completion-based (no withCheckedThrowingContinuation, no Task { @MainActor }).
• Native generated output contains 0 suspend fun and 0 async throws.
• Windows generated diff is additive only (operator<<, PigeonInternalToString).

Verification

• flutter analyze — clean
• flutter test — 285 passed
• flutter build macos --debug — built
• flutter build ios --debug --no-codesign — built
• example/android: ./gradlew :universal_ble:testDebugUnitTest (JDK 21) — BUILD SUCCESSFUL

Notes

• Windows was not built locally (no Windows toolchain); its generated interface is unchanged and CI covers flutter build windows + ctest.
• The bulk of the diff is generated code.