test(mcp): add async integration tests for prune_tools always_include, max_tools, early-return

## Missing tests

From test validation of PR #2293. These must be fixed before the wiring PR:

1. `always_include` partition logic — zero test coverage. Requires a mock `LlmProvider`.
2. `max_tools` cap — zero test coverage.
3. `min_tools_to_prune` early-return — current test is a tautology (never calls `prune_tools`).

## Component

`crates/zeph-mcp/src/pruning.rs`