[submit] Test coverage gaps: retry, HTTP 429, scheme rejection, hash parsing all untested

[obchain]

Refs #44

File: crates/charon-executor/src/submit.rs (tests)

Problem

Two unit tests cover default timeout value and syntactically invalid URL. Untested:

1. Timeout retry path (would require tokio::time::pause)
2. HTTP 429/503 classification — does it currently fall through to Err?
3. https-only scheme enforcement (after sibling fix lands)
4. Successful hash parse from JSON-RPC response
5. Reconnect on dropped connection

Live submission test is deferred per PR description, but the above are testable with httpmock or wiremock without a live endpoint.

Fix

Add httpmock to dev-dependencies. Add tests for each of the five scenarios above. Add tokio::time::pause coverage for the timeout path.

[obchain]

Addressed in commits d62d045 and e4170dc on feat/19-private-rpc-submit.