fix: preserve legacy protocol result serialization

[tsarlandie-oai]

Summary

• omit resultType: "complete" from responses for negotiated legacy sessions
• preserve required result discriminators for modern sessions and discovery
• keep negotiated session versions authoritative over conflicting request metadata
• expose protocol-aware ServerResult serialization and adaptation methods

Why

Result serialization applied the modern discriminator without accounting for the protocol version negotiated by the session. Legacy clients can reject that wire shape even though the underlying result is otherwise valid.

This keeps modern behavior intact while restoring the legacy-compatible response format for ordinary tools, lists, resources, historical discovery, and actual wire responses.

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 #1036