fix: guard stale Android CallInvite acceptance

[nhibberd]

Submission Checklist

• Updated the CHANGELOG.md to reflect the bug fix
• Tested code changes and observed expected behavior in the example app
• Performed a visual inspection of the Files changed tab for style consistency

All third-party contributors acknowledge that any contributions they provide will be made under the same open-source license that the open-source project is provided under.

• I acknowledge that all my contributions will be made under the project's license.

Description

Prevents an Android native null-pointer crash when a CallInvite is cancelled after JavaScript calls accept() but before the queued main-thread acceptance runs.

CallInviteModuleProxy.getCallRecord() currently validates the invite before posting work to the main thread. A cancellation can remove or settle that invite before VoiceService.acceptCall() serializes its custom parameters, causing CallInvite.getCustomParameters() to be called through a null reference. JavaScript cannot catch this native main-thread crash.

This restores the intent of #695 after its CI-related revert in #699, while also closing the remaining race at the main-thread boundary and making invite serialization null-safe.

Breakdown

• Resolve and validate the call record inside the main-thread runnable.
• Reject acceptance when the invite is missing or no longer active.
• Cache the validated invite for acceptance so concurrent cancellation cannot turn a later read into null.
• Return empty custom parameters when a cancelled incoming invite is serialized.

Validation

• yarn check
• 523 Jest tests passed with 100% reported coverage.
• TypeScript and ESLint checks passed.
• Android Gradle compilation was not run locally because the test machine has no Java runtime.

Additional Notes

This is an internal defensive change with no public API changes.