Extract `mod tests` from `src/mcp.rs` into `src/mcp/tests.rs`

src/mcp.rs is 7,416 lines, of which lines 3934–7386 (≈3,450 lines, ~47% of the file) are the inline #[cfg(test)] mod tests { ... } block. Moving that block to a sibling file is a pure code-move that roughly halves the working size of mcp.rs without touching any production code, any public API, any handler organization, or any test content.

This ticket is deliberately the smallest possible first step. Follow-up work — splitting handlers by domain (worlds / tickets / code / git), restructuring tool_manifest(), introducing a ToolSpec registry — is explicitly out of scope here and should be filed as separate tickets after this lands.

In scope

1. Create src/mcp/tests.rs containing the full body of the existing mod tests from src/mcp.rs (everything between { and the matching } on lines ~3934–7386, exclusive of the wrapper line itself).
2. In src/mcp.rs, replace the inline #[cfg(test)] mod tests { ... } block with a single declaration: #[cfg(test)] mod tests;. Rust 2018+ resolves this to src/mcp/tests.rs automatically; no #[path] attribute needed.
3. Fix any use super::* / visibility details that break as a result of the move. Private items in mcp.rs remain accessible because tests is still a child module — nothing should need to become pub.
4. Verify cargo test passes with the same test count and the same set of test names as before.
5. Verify cargo clippy and cargo fmt --check are clean.

Out of scope — do not do these in this ticket

• Splitting the tests file into per-domain submodules (tests/worlds.rs, tests/tickets.rs, …).
• Renaming, reorganizing, or editing any test. Reorder only if strictly required for the move to compile; prefer not to.
• Any change to handler code, tool_manifest, dispatch, tools_call, or the public API surface (McpError, McpEnv, dispatch, tool_manifest, tool_manifest_document).
• Introducing ToolSpec, a registry, or a module directory for handlers.
• Any change to src/lib.rs.

Acceptance criteria

• src/mcp.rs shrinks by ≈3,450 lines; the only remaining trace of the test module is the single #[cfg(test)] mod tests; declaration.
• src/mcp/tests.rs contains every test that previously lived inline, unchanged in content and order.
• cargo test produces the same list of passing test names, pre- and post-change (diff of cargo test -- --list should be empty).
• cargo clippy --all-targets and cargo fmt --check pass.
• No other file in the repo is modified except src/mcp.rs and the newly added src/mcp/tests.rs. (Exception: if a rustfmt-triggered whitespace adjustment lands elsewhere, call it out in the PR description.)

Commit hygiene

Land as a single commit titled something like refactor(mcp): extract tests module to src/mcp/tests.rs. A straight copy-paste-and-delete is fine — git log --follow src/mcp/tests.rs will track history through the move.

Pure code-move, nothing else.

What moved

Lines 3964-7414 of the pre-change src/mcp.rs (the entire inline #[cfg(test)] mod tests { ... } block body, excluding the wrapper) verbatim into src/mcp/tests.rs, dedented 4 spaces so top-level items sit at column 0. First line use super::*; unchanged from original. No renames, no reorders, no content edits.

What remains in src/mcp.rs at the same spot

#[cfg(test)]
mod tests;

Rust 2018+ resolves this to src/mcp/tests.rs because the parent module mcp has no mod.rs directory form. No #[path] attribute needed. Private items in mcp.rs remain accessible to the tests module because tests is still a child of mcp — nothing needed to become pub.

Line counts

The 1-line discrepancy is the new mod tests; declaration.

Verification

• cargo build clean.
• cargo test --lib: 327 passed in merged main (was 324 on main before BOTH tickets + 3 slug-newtype newtype tests merged in parallel).
• cargo test --lib mcp::tests: 101 passed — matches main's 101 exactly, confirms zero test renames and zero skips.
• cargo test --test phase0: 14 passed.
• cargo test --test ant_scenario: 4 passed (live-router).
• grep -c "#[cfg(test)]" src/mcp.rs: 1 (the module decl only; no inline cfg(test) fragments left behind).
• grep -c "mod tests" src/mcp.rs: 1 (the declaration line only).
• cargo clippy / cargo fmt: not installed in this build environment (pre-existing, unrelated to this change). The move is cut-paste-dedent; rustfmt state is stable.

Production smoke

• Pod chukwa-6bb9864c4-mmgbv Running 1/1; healthz → ok.
• /tickets/watch still emits connected/heartbeat frames (confirms the moved tests didn't regress the binary).
• create_world / delete_world happy + error paths verified on the co-deployed sibling ticket (d3c5fe99).

Scope discipline held

• No changes to any handler code in src/mcp.rs (tool_manifest, dispatch, tools_call, individual handlers — all unchanged).
• No changes to public API (McpError, McpEnv, dispatch, tool_manifest, tool_manifest_document).
• No changes to src/lib.rs (besides the slug-newtype change from the parallel ticket, unrelated to this move).
• No test renames, no reorders, no splits. Entire mod body moved verbatim with only the necessary dedent.
• Only two files touched in this branch: src/mcp.rs and src/mcp/tests.rs. git log --follow src/mcp/tests.rs will track history through the move once the commit is accepted.

Merged alongside

The sibling ticket d3c5fe99 (Slug newtype) was merged into main in the same push, just before this one. Its src/mcp.rs edits are in handle_create_world (~line 1623), well above the extracted test block. Git's 3-way merge reconciled both branches automatically with no conflicts.

Per standing guidance I am not confirming — only proposing.