Add request body redaction and size limits

[craigpotter]

Overview of change

Closes the request/response asymmetry from the review pass: responses have excluded_response_body and max_response_size, but request bodies were always stored in full — login payloads, personal data, large uploads and all.

• excluded_request_body — mirrors excluded_response_body: '*', connector classes, or request classes; matching bodies are stored as REDACTED. Empty by default (no behaviour change).
• max_request_size — mirrors max_response_size: kilobytes, measured in bytes, default 100. Oversized bodies are stored as <Unsupported Barstool Request Content>.
• No content-type gate on requests (unlike responses) — the application authored the body, so it is always loggable text.
• Request bodies are now always persisted as strings. Saloon's standard body repositories are all Stringable; custom repositories that are not fall back to json_encode($body->all()) (previously a non-Stringable repository would have crashed the PDO insert).
• Streamed and multipart placeholders unchanged.

README updated with matching sections; new JsonPostRequest test fixture.

Testing

Six new tests: JSON body stored as string, redaction via '*'/connector/request class, byte-measured size cap with a multibyte body, and null body passthrough. Full suite: 53 passed (191 assertions), PHPStan level 8 clean, Pint clean.