[Java] Tool-as-lambda 4.4: Add unit tests for API behavior and validation

[edburns]

Overview

Implement Phase 4.4 of #1810: add focused unit tests for lambda tool API behavior, options propagation, and validation paths.

Before coding

• Do not start any implementation work until this issue is explicitly assigned to the coding agent.
• Base branch for this work: edburns/1810-java-tool-ergonomics-tool-as-lambda.
• Read this plan file on the branch before coding:
  1810-java-tool-ergonomics-tool-as-lambda-remove-before-merge/1810-ignorance-reduction-for-implementation-plan.md

Ordering rule (important)

This issue starts only after 4.3 is merged; all Phase 4 issues are completed in listed order.

Relevant Phase 3 resolutions to apply

• 3.1 / 3.2: assert resolved overload families and lambda call-site behavior.
• 3.3: test Param<T> validation semantics and required/default interactions.
• 3.5: verify coercion behavior is consistent with existing ObjectMapper policy.
• 3.6: verify fluent option modifiers (overridesBuiltInTool, skipPermission, defer) propagate correctly.
• 3.7: verify construction-time validation failures and message quality.

Scope / deliverables

Add deterministic tests under java/src/test/java/com/github/copilot/tool/ and/or closely-related existing test packages covering:

1. Successful inline definitions across supported arities.
2. Sync and async handlers.
3. Option flag propagation.
4. Required/default semantics.
5. Error/validation paths.

Gating criteria

• Tests cover both success and failure paths and are deterministic.
• Coverage is sufficient to protect the 4.2/4.3 contract.

Constraints

• Keep tests focused on API behavior and validation for lambda tools.
• Avoid unrelated test churn.

Completion bookkeeping

When complete, update the checkbox for Phase 4.4 in:
1810-java-tool-ergonomics-tool-as-lambda-remove-before-merge/1810-ignorance-reduction-for-implementation-plan.md