fix: conservatively retry legacy server initialization

[tsarlandie-oai]

Summary

• classify discovery-probe failures with typed HTTP, JSON-RPC, and protocol-version signals
• retry legacy initialization only for correlated method-not-found responses, known historical-version evidence, recognized legacy prevalidation responses, and initial HTTP 404/405
• preserve typed 401/403 authentication failures and avoid fallback for arbitrary, contradictory, unknown, or unrelated errors
• replace string matching on transport errors with UnexpectedHttpStatus { status, body }
• keep the HTTP worker alive for eligible initial 404/405 probes so Auto mode can perform the legacy handshake

Why

Automatic fallback is useful only when the failed discovery probe demonstrates that the endpoint is legacy. Broad error matching risks retrying initialization after authentication failures, unrelated responses, or future protocol behavior.

The rework separates failure extraction from the fallback policy and makes each accepted legacy signal explicit and testable.

Validation

• lifecycle classifier and integration suite: 17 tests passed
• HTTP discovery/fallback suite: 10 tests passed
• streamable HTTP error-body suite: 3 tests passed
• cargo +1.95.0 clippy -p rmcp --all-features --all-targets -- -D warnings
• formatting and whitespace checks passed

Fixes #1040