Fix Auth APNs token timeout for concurrent waiters

[Hokila]

Motivation

AuthAPNSTokenManager starts one timeout for the first caller and coalesces later callers into the same pending batch. The timeout previously compared callback counts, so adding a second waiter changed the count and prevented the timeout from firing. The corresponding async getToken() continuations could then wait indefinitely.

Related: #13470

Changes

• Track the active APNs registration batch with a request generation.
• Complete every callback in the active batch when the shared timeout expires.
• Ignore stale timeouts after cancellation or completion of an earlier batch.
• Add callback-level, stale-timeout, and concurrent async waiter regression tests.
• Document the fix in the FirebaseAuth changelog.

Non-goals

• No changes to public APIs.
• No changes to the separately identified FirebaseAuth error! sites or Storage/Database TODOs.

Validation

• xcodebuild -scheme AuthUnit -destination 'platform=iOS Simulator,id=B33A771B-CD3A-41BA-81ED-2CFEE30AEA14' -only-testing:AuthUnit/AuthAPNSTokenManagerTests CODE_SIGNING_ALLOWED=NO test — 8 tests passed.
• xcodebuild -scheme AuthUnit -destination 'platform=iOS Simulator,id=B33A771B-CD3A-41BA-81ED-2CFEE30AEA14' CODE_SIGNING_ALLOWED=NO test — 354 tests passed.
• ./scripts/check.sh --allow-dirty --test-only upstream/main — passed.
• git diff --check — passed.

API / release impact

Internal correctness fix only; no public API or compatibility changes.

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[paulb777]

LGTM. Thanks for the PR!

[paulb777]

Please fix the unit test build issue:

  - ERROR | [iOS] xcodebuild:  /Users/runner/work/firebase-ios-sdk/firebase-ios-sdk/FirebaseAuth/Tests/Unit/AuthAPNSTokenManagerTests.swift:234:46: error: implicit capture of 'self' requires that 'AuthAPNSTokenManagerTests' conforms to 'Sendable'

[Hokila]

Fixed in the latest commit and verified with CocoaPods lint on Xcode 26.4. Thanks!