[Java] Add Param<T> public API type for lambda-defined tools

[Copilot]

Phase 4.1 of #1810: introduces the runtime parameter metadata class needed by the upcoming ToolDefinition.from(...) lambda overloads.

Changes

• Param<T> (com.github.copilot.tool) — immutable, fluent parameter descriptor with construction-time validation:
  • Rejects blank name/description
  • Rejects required=true combined with a non-empty defaultValue
  • Validates defaultValue against declared Class<T> (String, numeric primitives/boxed, Boolean, enums; unsupported types throw)
• ParamTest — 27 unit tests covering factory methods, fluent mutators, all validation rules, and equality semantics
• Plan file — Phase 4.1 checkbox marked complete

Usage

// Required param, no default
Param<String> query = Param.of(String.class, "query", "Search query text");

// Optional param with validated default
Param<Integer> limit = Param.of(Integer.class, "limit", "Max results", false, "10");

// Fluent mutation (returns new instance)
Param<String> renamed = query.name("search_query");
Param<String> optional = query.defaultValue("*"); // implicitly sets required=false

No changes to module-info.java needed — com.github.copilot.tool is already exported.