Add ListTools RPC and refactor tool metadata

[begelundmuller]

This PR:

• Adds a ListTools RPC that returns metadata about all registered AI tools
• Updates some tool descriptions, notably adding meta["openai/toolInvocation/invoking"] and meta[""openai/toolInvocation/invoked""] to all tools

RPC schema:

// ListTools lists metadata about all AI tools that calls to Complete(Streaming) may invoke.
// Note that it covers all registered tools, but the current user may not have access to all of them.
rpc ListTools(ListToolsRequest) returns (ListToolsResponse) {
  option (google.api.http) = {get: "/v1/instances/{instance_id}/ai/tools"};
}

// Request message for RuntimeService.ListTools
message ListToolsRequest {
  string instance_id = 1;
}

// Response message for RuntimeService.ListTools
message ListToolsResponse {
  repeated rill.ai.v1.Tool tools = 1;
}

You can view the RPC output directly at:

http://localhost:9009/v1/instances/default/ai/tools

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[ericpgreen2]

Looks good 👍