fix: Safely handle empty model in GoogleSearchTool request processor

[hemasekhar-p]

1. Link to an existing issue (if applicable):

• Closes: [BUG] GoogleSearchTool.processLlmRequest throws NoSuchElementException synchronously on an empty model #1279

Problem:
GoogleSearchTool.processLlmRequest uses an unguarded Optional.get() to extract the model llmRequestBuilder.build().model().get(). If the model is absent, this throws a NoSuchElementException,

Solution:
Replaced the .get() call with .orElse(null) in GoogleSearchTool.java. By defaulting to null, the method's existing error-handling logic takes over and gracefully returning a Completable.error(IllegalArgumentException) and ensuring errors flow correctly through the Completable stream.

Testing Plan

Unit Tests:

• I have added or updated unit tests for my change.
• All unit tests pass locally.

Added testProcessLlmRequest_WithNoModel_DoesNotThrowsException to BaseToolTest.java. The test utilizes assertDoesNotThrow to verify that invoking processLlmRequest with a null model no longer triggers a NoSuchElementException. All tests in BaseToolTest pass successfully locally.

Checklist

• I have read the CONTRIBUTING.md document.
• My pull request contains a single commit.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have manually tested my changes end-to-end.
• Any dependent changes have been merged and published in downstream modules.

[hemasekhar-p]

@anFatum, Could you please review this PR.