Increase gRPC max metadata size and optimize integration test CI runs

[cherylEnkidu]

Summary

Fixes Firestore Nightly CI failures and timeouts.

Changes

• Increase gRPC max metadata size (grpc_connection.cc): Set GRPC_ARG_MAX_METADATA_SIZE to 1 MB (up from 8 KB) so large error trailers on nightly/staging backends do not trigger RESOURCE_EXHAUSTED retry loops.
• Use regional nightly endpoints (FSTIntegrationTestCase.mm): Route nightly tests to nam5-test-firestore.sandbox.googleapis.com (standard) and us-central1-test-firestore.sandbox.googleapis.com (enterprise) to work around UberProxy auth blocking test-firestore.sandbox.googleapis.com (b/552991017).
• Fix integration test stability (FIRDatabaseTests.mm, FSTDatastoreTests.mm, FIRServerTimestampTests.mm): Pass [FSTIntegrationTestCase databaseID] instead of defaulting to (default), and read documents after transaction commit in server timestamp tests instead of waiting on a pre-existing listener.
• Optimize CI build & retries (scripts/build.sh): Skip redundant full-suite xcodebuild retries when -retry-tests-on-failure is enabled, and increase build concurrency from -jobs 4 to -jobs 8.

#no-changelog

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