feat(bigquery-jdbc): add TelemetryManager singleton foundation and exception safeguards

[Neenu1995]

Introduces the internal package-private TelemetryManager singleton hub for PR 7 of the client-side diagnostic telemetry client implementation (b/527947900).

• Singleton Lifecycle: Implements thread-safe double-checked locking (volatile) for zero-contention fast-path telemetry checks (getInstance(), init(), closeInstance()).
• Driver Stability Safeguard: Adds exception-isolation wrapper runSafely(Runnable) ensuring telemetry operations never throw or block driver connection/query hotpaths.
• Unit Testing: Adds TelemetryManagerTest covering singleton initialization contract, double-checked locking, custom configuration overrides, and zero-exception bubbling.

[gemini-code-assist]

Code Review

This pull request introduces a thread-safe singleton, TelemetryManager, to manage the lifecycle of client-side diagnostic and usage telemetry in the BigQuery JDBC driver, along with corresponding unit tests and minor .gitignore updates. The review feedback suggests improving the singleton's lifecycle management by introducing a DISABLED_INSTANCE sentinel. This sentinel prevents re-initialization with default settings when telemetry is explicitly disabled, and the feedback includes specific suggestions to update getInstance(), init(), isInitialized(), and closeInstance() to handle this sentinel state correctly.

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud