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

[edburns]

Requirement: When you open the PR, the very first thing you put in the description must be Fixes #1839 where the issue number is this issue for which the PR aims to implement the work.

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 IMPORTANT: you must run mvn spotless:apply in the java directory before every commit. - 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