[Java] Tool-as-lambda 4.1: Add public API types for lambda-defined tools

[edburns]

Overview

Implement Phase 4.1 of #1810: add the public API types needed to define tools inline with lambdas.

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)

Issues for Phase 4 are resolved strictly in plan order (4.1, then 4.2, ...). Each issue is merged to edburns/1810-java-tool-ergonomics-tool-as-lambda before the next one starts.

Relevant Phase 3 resolutions to apply

• 3.1 API shape: v1 sync/async families are from, fromAsync, plus context-aware fromWithToolInvocation / fromAsyncWithToolInvocation.
• 3.2 functional interfaces: use JDK functional interfaces (Supplier/Function/BiFunction), no custom SAM interfaces in v1.
• 3.3 parameter DSL: use runtime metadata class Param<T> with fluent API and validation.
• 3.8 package placement: user-facing helper types belong in com.github.copilot.tool; avoid leaking internal helper types.

Scope / deliverables

1. Add/finish public API type(s) under java/src/main/java/com/github/copilot/tool/ required by lambda tool definitions, including Param<T>.
2. Ensure the API is discoverable and unambiguous at call sites.
3. Ensure module exports/public surface remain minimal and user-facing only.

Validation and behavior requirements

• Param rejects blank name/description.
• required=true with non-empty default is rejected.
• default values are validated against declared Class<T>.
• Public signatures compile cleanly and are ready for Phase 4.2 overload implementation.

Constraints

• Keep changes focused to this sub-item only.
• Follow existing Java style and public API documentation conventions.

Completion bookkeeping

When complete, update the checkbox for Phase 4.1 in:
1810-java-tool-ergonomics-tool-as-lambda-remove-before-merge/1810-ignorance-reduction-for-implementation-plan.md