fix: accept namespaced discovery server information

[tsarlandie-oai]

Summary

• move discovery server identity to the canonical _meta["io.modelcontextprotocol/serverInfo"] key
• treat server identity as optional, display-only data and expose an anonymous-result constructor
• accept legacy top-level serverInfo during deserialization, then normalize it to canonical metadata
• prefer the canonical metadata key when both representations are present
• preserve malformed identity metadata without rejecting an otherwise valid discovery response
• represent client peer information with ServerPeerInfo, where server_info is optional across discovery and legacy initialization

Why

The discovery schema carries server implementation identity as optional result metadata. Requiring a top-level serverInfo made conforming anonymous responses fail and made the client peer-info type unable to represent discovery accurately.

The compatibility decoder keeps accepting the earlier top-level shape, while serialization now emits only the canonical metadata representation.

Compatibility

This changes RoleClient::PeerInfo from ServerInfo to ServerPeerInfo. Legacy initialization converts losslessly and always supplies Some(server_info); discovery clients must handle an absent identity.

Validation

• discovery and lifecycle integration suites: 18 tests passed
• canonical, legacy, precedence, malformed, and anonymous identity cases covered
• cargo +1.95.0 clippy -p rmcp --all-features --all-targets -- -D warnings
• formatting and whitespace checks passed

Fixes #1039