java: enforce non-blank @CopilotToolParam description at compile time

[rinceyuan]

Closes #1836

Summary

Adds compile-time validation in CopilotToolProcessor to reject @CopilotToolParam annotations with blank value() (description). Without a meaningful description, the LLM cannot reliably select and invoke tools.

Changes

CopilotToolProcessor.java

• Added blank-description check after the single-record wrapper validation block
• Checks paramAnnotation.value().isBlank() (catches both empty and whitespace-only)
• Single-record wrapper parameters are exempt (as specified in the issue)
• Error message identifies the parameter name, enclosing method, and class

CopilotToolProcessorTest.java (5 new tests)

1. Blank value — @CopilotToolParam("") produces compile error
2. Whitespace-only value — @CopilotToolParam(" ") produces compile error
3. Valid value — @CopilotToolParam("Search query") compiles without error
4. No annotation — unannotated parameter compiles without error
5. Record wrapper exemption — @CopilotToolParam on single-record wrapper is exempt

Error message example

error: @CopilotToolParam on parameter 'query' in 'BlankDescTools.searchItems' has a blank value (description).
Descriptions are required so the LLM can correctly select and invoke the tool

Testing

• mvn test -Dtest=CopilotToolProcessorTest — 39 tests pass (0 failures)
• All existing tests pass with no regressions
• Local JDK is 17; CI will run the full mvn verify with JDK 25+

[rinceyuan]

@microsoft-github-policy-service agree company=Microsoft

[rinceyuan]

@edburns Hi! This implements #1836 — compile-time validation for blank @CopilotToolParam descriptions in the annotation processor. 5 new tests, all passing. Would appreciate your review. Thanks!