Support generic ACP elicitation/create for structured input

[whats0Wong]

Summary

• Forward generic MCP form/url elicitation requests to ACP clients via elicitation/create when the client advertises matching elicitation capabilities.
• Preserve the existing MCP tool approval mapping to session/request_permission.
• Convert ACP elicitation responses back into Codex Op::ResolveElicitation, including action, content, and _meta.
• Add coverage for both form-mode and url-mode forwarding.

Notes

• Unsupported generic elicitations continue to be auto-declined.
• RFC alignment boundary: this PR implements the elicitation/create form/url forwarding path and capability-gated behavior. It does not send ACP elicitation/complete notifications yet, because the current Codex generic MCP elicitation event/Op flow exposed to codex-acp provides ElicitationRequest + ResolveElicitation, but not a separate URL out-of-band completion hook. Therefore URL-mode user consent is handled through the elicitation/create response, while a future upstream completion signal would be needed for a fully accurate elicitation/complete bridge.

Tests

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check

Closes #294

Real validation with helix

Validated the generic form and url elicitation paths against code.byted.org/devinfra/helix on origin/feat/acp-elicitation-worker using a real local codex-acp process over ACP stdio and a real MCP stdio server.

The validation confirmed the full create/resolve flow: initialize advertises clientCapabilities.elicitation.form and clientCapabilities.elicitation.url, session/new and session/prompt succeed, the MCP server issues generic elicitation/create, codex-acp forwards ACP elicitation/create, helix emits WorkerSignalKindElicitationRequest, helix ResolveElicitation replies with accept, and Codex completes the MCP tool call/end turn.

This also exposed and this PR now fixes the real-event routing case where Codex emits MCP generic elicitation with event id mcp_elicitation_request instead of the prompt submission id.