fix: conservatively retry legacy server initialization

[tsarlandie-oai]

Summary

• retry legacy initialization for correlated method-not-found errors and known historical-only protocol signals
• handle observed legacy HTTP 400 prevalidation responses and initial 404/405 responses
• avoid fallback for unrelated IDs, arbitrary null-ID errors, contradictory or unknown versions, authentication failures, and existing-session 404 responses
• preserve existing legacy initialization error handling

Why

Automatic legacy initialization needs to recognize genuine historical server behavior without turning unrelated protocol, authentication, or active-session failures into unsafe retries.

This narrows fallback to signals that reliably indicate a legacy endpoint and explicitly rejects ambiguous or contradictory cases.

Validation

• comprehensive regression suite: 516 tests passed
• whole-workspace formatting check passed
• all-target, all-feature Clippy passed with the documented beta-only toolchain warning suppressed

Fixes #1040