Fix synchronous throttled request failures

[arnabnandy7]

Summary

Add top-level error handling around onThrottleReady() request setup for:

• CQL requests
• prepare requests
• graph requests
• continuous paging requests

Fixes #949.

Root cause

Request throttlers invoke onThrottleReady() directly. Query-plan creation, execution-profile
resolution, metric updates, or request scheduling can throw synchronously from this callback.

Those exceptions previously bypassed the request handlers' normal terminal-error paths. On the
immediate throttler path, this could make executeAsync() throw instead of returning a failed
future. On a queued path, the exception could escape on the throttler or event-loop thread and
leave the request future incomplete.

Solution

Wrap each request handler's onThrottleReady() setup in a catch-all error boundary and route
failures through its existing terminal-error flow.

This ensures that synchronous setup failures:

• complete the user-facing future exceptionally;
• release or notify the request throttler correctly;
• behave consistently for immediate and delayed requests.

A regression test verifies that a synchronous query-plan creation failure is returned through a
failed completion stage instead of escaping request construction.

Testing

• CqlRequestHandlerTest: 7 tests passed
• CqlPrepareHandlerTest: 9 tests passed
• GraphRequestHandlerTest: 30 tests passed
• ContinuousGraphRequestHandlerTest: 5 tests passed
• Maven formatting and XML checks passed
• git diff --check passed

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: b1590bb0-8eda-4ef0-9087-861ba6e17968

📥 Commits

Reviewing files that changed from the base of the PR and between 3cafb2a and 4c67775.

📒 Files selected for processing (5)

• core/src/main/java/com/datastax/dse/driver/internal/core/cql/continuous/ContinuousRequestHandlerBase.java
• core/src/main/java/com/datastax/dse/driver/internal/core/graph/GraphRequestHandler.java
• core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareHandler.java
• core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlRequestHandler.java
• core/src/test/java/com/datastax/oss/driver/internal/core/cql/CqlRequestHandlerTest.java

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📝 Walkthrough

Walkthrough

The request handlers for CQL, prepare, graph, and continuous CQL now catch exceptions thrown during throttled request setup, including execution-profile resolution, query-plan construction, metrics updates, and initial dispatch. Failures are routed through existing final-error or abort paths, and continuous requests also signal the throttler. A CQL test verifies that a query-plan creation exception completes the returned future exceptionally with the same exception.

Suggested reviewers: nikagra

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main change: adding synchronous error handling for throttled request setup.
Description check	✅ Passed	The description is on-topic and accurately describes the throttled request error-handling fix.
Linked Issues check	✅ Passed	The changes address #949 by wrapping onThrottleReady setup in error boundaries across CQL, prepare, graph, and continuous handlers.
Out of Scope Changes check	✅ Passed	The changed files and added test all support the throttled request failure fix, with no clear unrelated additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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