feat: add exact raw JSON request handling

[steipete]

Summary

• add RawJSONValue and ExactJSONObject for ordered members, duplicate keys, exact Unicode-scalar identity, exact number lexemes, and strings that never auto-decode as data URLs
• add a source-compatible two-argument Server.withMethodHandler overload that receives the normal typed request alongside RawRequestContext
• route raw-aware single and batch requests with zero-copy, cancellation-aware parsing and explicit document/string/depth/entry limits while leaving legacy handlers, notifications, and inbound responses on their existing decoding path
• keep malformed batch documents as top-level parse errors while isolating structurally invalid or over-limit items so valid siblings still run and available request IDs are retained
• preflight programmatic data URL sizes with overflow-checked base64 length arithmetic before allocation

Validation

• swift test (575 tests on macOS)
• swift build -c release
• swift package generate-documentation --target MCP --warnings-as-errors
• Swift 6.1.2 Linux: swift test (494 tests) and swift build -c release
• Swift 6.1.2 static Linux SDK: upstream install-and-build-with-sdk.sh --static

Documentation

• document exact JSON method handlers and custom RawJSONLimits in the README