feat(security): limit request body size

[oxura]

Closes #341.

Request bodies are currently unbounded at the project boundary. An upload or chunked request can therefore keep growing until the process runs out of memory.

This adds a global 2 MiB default and a max_request_body_size TOML setting for projects that need a different ceiling. Every incoming body is wrapped before it reaches routing or middleware, and the limit counts bytes across the full stream rather than trusting Content-Length.

When the limit is crossed, the body returns a Cot error with HTTP 413. The error keeps the normal project error path intact, so production uses the configured error handler; in debug diagnostics the message points directly at max_request_body_size. Existing body errors also keep their original Cot status instead of being flattened into a 400 while crossing the wrapper.

Coverage includes the exact boundary, a multi-chunk overflow, TOML/default configuration, and an end-to-end project test proving a custom error handler receives and returns the 413.

Checked with:

• cargo test -p cot_core body::tests (8 passed)
• cargo test -p cot --all-features request_body_limit (2 passed)
• cargo clippy -p cot_core -p cot --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• git diff --check