Respond with 414 when a line exceeds the maximum line length

[trevorturk]

Fixes #237.

Previously, when a request line or header line exceeded maximum_line_length, LineLengthError propagated past next_request (it doesn't include the BadRequest marker), so the server closed the connection without writing anything and the peer saw an unexplained EOF.

This PR rescues LineLengthError in next_request and uses the existing fail_request path to respond with 414 before re-raising, matching the existing BadRequest → 400 handling. The request-line and header-line cases can't be distinguished at the rescue site, so both get 414 as the more common case — happy to split the error by phase in protocol-http1 and map 414/431 instead if you'd prefer that precision.

Types of Changes

• Bug fix.

Contribution

• I added tests for my changes.
• I tested my changes locally.
• I agree to the Developer's Certificate of Origin 1.1.