fix(#10): emit ToolCall events for meta-tool invocations

[doobidoo]

Closes #10.

Summary

handle_meta_find_tools and handle_meta_list_categories previously emitted no EventType::ToolCall, so invocations through those dispatch paths never landed in /api/metrics. Clients whose MCP flow is biased toward mcplex_find_tools (e.g. Claude Desktop/Code via the stdio bridge on macOS) would see a permanently empty Tool Statistics dashboard pane.

handle_meta_call_tool already routes through dispatch_real_tool, which records under the real upstream tool name — left alone.

Changes

• src/protocol/transport.rs:
  • handle_meta_find_tools: start-time capture at function entry. EventType::ToolCall emitted on the success return path. Also emitted with success: false on the empty-query error branch so error rates don't silently hide.
  • handle_meta_list_categories: same pattern for the success return (no user-input error branch here).
  • Both events set tool_name = "<meta_tool_name>", server_name = "mcplex".

Verified

• cargo build --release — clean
• cargo fmt --all -- --check — clean
• Tested live (will follow up with a repro transcript in a comment)

No behavior change for mcplex_call_tool, no schema change, no config change. Minimum-surface fix for the reported regression.

[doobidoo]

Live verification

Built PR branch locally and ran against a minimal config (/tmp/mcplex-test.toml: port 3199 gateway / 9099 dashboard, one context-provider backend, cache disabled, RBAC off). Issued three calls and diffed /api/metrics before/after.

```
=== BEFORE ===
tools: []

=== CALLS ===
mcplex_find_tools {"query":"instincts"} -> OK
mcplex_list_categories {} -> OK
mcplex_find_tools {"query":""} -> ERR (expected, empty-query)

=== AFTER ===
tools:
{ name: "mcplex_list_categories",
invocations: 1, successes: 1, errors: 0 }
{ name: "mcplex_find_tools",
invocations: 2, successes: 1, errors: 1 }
```

Matches the expected behavior from the PR description:

• handle_meta_list_categories success path emits a ToolCall event → 1 invocation / 1 success / 0 errors. ✅
• handle_meta_find_tools success path emits on valid query → 1 success. ✅
• handle_meta_find_tools empty-query error branch emits with success=false → +1 invocation / +1 error on the same mcplex_find_tools tool row. ✅

The dashboard's Tool Statistics section now populates for any client that drives the gateway through meta-tool discovery (which is the dominant pattern on macOS via the stdio bridge). Closes #10.