test(server): hold every MCP answer to McpJson on the raw body

[youndie]

Why

katcher's ContentNegotiation (common()) pretty-prints and does not write defaults. On MCP SDK 0.15.0 the one thing the stateless transport sends through the application's ContentNegotiation is the answer to a POST in JSON mode, and under katcher's Json that answer loses protocolVersion from initialize and the whole result from ping. kore-mcp encodes the answer with McpJson before ContentNegotiation sees it — and the only test here watching that looked at protocolVersion alone.

What changes

• McpWireFormatTest (commonTest): ten exchanges through the assembled module — initialize latest and older, ping, tools/list, tools/call without arguments, a call ending in a tool error, an unknown tool, an unknown method, a batch of two, an unparseable body. Each raw body must be exactly what McpJson writes for the message inside it.
• Positive control in the same class: katcher's common(), katcher's five tools, the same requests, the transport installed by the bare SDK. It must come out different; the KDoc says what to do if it ever stops.
• McpEndpointTest: the protocolVersion test is removed — the new test catches the missing field (see below); the negotiated value itself is kore-mcp's and is tested there.
• README, MCP section: why the SDK's ContentNegotiation is already installed… warning at startup is expected and what holds it. CLAUDE.md: one sentence pointing at the test.

Verified

Locally, JVM and the macOS native target: :server:jvmTest and :server:nativeTest --tests 'io.github.youndie.katcher.mcp.*' green, ktlintCheck clean. Linux native is CI's build.

• Mutation: installMcp switched from installKoreMcp to the bare SDK's mcpStatelessStreamableHttp → the new test fails with 9 of 10 exchanges different (all but the unparseable body, which the SDK writes itself); the removed protocolVersion test failed on the same mutation.
• One-off probes (not committed), bare SDK with katcher's tools: under katcher's Json 9 of 10 differ; under McpJson none; under katcher's Json without prettyPrint three still do — initialize latest, ping and the batch carrying it, none of which McpJson can read back. So the comparison catches the dropped protocolVersion on its own, not only whitespace.
• The warning is in the stand's startup log, as the README now says.