[Java] Tool-as-lambda 4.3: Implement schema + coercion internals for Param<T>

[edburns]

Overview

Implement Phase 4.3 of #1810: build the internal schema/coercion layer that maps Param<T> metadata and invocation arguments to runtime behavior.

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 4.2 is merged; all Phase 4 issues are completed in listed order.

Relevant Phase 3 resolutions to apply

• 3.4 type mapping scope: support exactly the existing Java schema/tool parameter-type surface; no new schema semantics.
• 3.5 coercion policy: reuse the existing ObjectMapper conversion policy (do not invent bespoke conversion paths that diverge from existing behavior).
• 3.7 validation boundaries: validate at construction time, including duplicate parameter names and unsupported mappings, with precise exceptions.
• 3.8 package boundaries: place user-visible vs internal helpers appropriately; do not leak internal-only types.

Scope / deliverables

1. Implement internal helper(s) (likely under java/src/main/java/com/github/copilot/tool/) that:
   • produce JSON schema from declared Param<T> metadata
   • coerce invocation arguments into typed lambda arguments consistently
2. Ensure behavior aligns with baseline contract verified in Phase 2 and consumed by 4.2 overloads.

Gating criteria

• Internal mapping/coercion behavior matches existing Java tool behavior contract.
• Clear failure messages for invalid definitions.

Constraints

• Keep scope limited to internals for schema/coercion.
• Do not broaden supported type semantics beyond existing pipeline.

Completion bookkeeping

When complete, update the checkbox for Phase 4.3 in:
1810-java-tool-ergonomics-tool-as-lambda-remove-before-merge/1810-ignorance-reduction-for-implementation-plan.md