codegen: emit MCP and A2A agent surfaces from protocol bundle

[bokelley]

Implement generated MCP/A2A agent-surface emission from the signed protocol bundle.

Scope:

• Generate MCP tool descriptions from protocol bundle tool/field text.
• Keep emitted MCP tool names, A2A skill IDs, and JSON schema names protocol-stable even when Java names follow Java idiom.
• Emit universal request fields such as idempotency_key on every relevant tool input.
• Set MCP idempotency annotations such as idempotentHint: true where supported and where the AdCP request surface accepts idempotency.
• Follow a cross-SDK-stable A2A skill ID convention so Java and TS agents dedupe in registries.
• Add snapshot coverage for generated MCP tool descriptors and A2A skill metadata in the v0.1 conformance runner.

References:

• PR docs: update roadmap for AdCP 3.1 parity #23
• ROADMAP.md Track 2 / Track 3 / Track 9
• specs/agent-surface-emission.md