docs: clarify default legacy initialize and SdkHttpError on connect

[luquitas-cloud]

Summary

Documents the residual DX points from #2410 after the dual-era / retry claims were narrowed:

• Default client connect still uses legacy initialize (not a dual-era probe). Opt into versionNegotiation: { mode: 'auto' } for modern-first behavior.
• Endpoints that never implement the MCP lifecycle are not MCP servers; the SDK will not connect them.
• Non-2xx responses to initialize surface as SdkHttpError: Error POSTing to endpoint: ... (JSON-RPC fields live in the body text / error.data.text), while a JSON-RPC error on HTTP 200 becomes ProtocolError.
• Failed initialize POSTs are not retried three times by the SDK.

Changes

• docs/protocol-versions.md — explicit default wording + tip for non-MCP JSON-RPC endpoints
• docs/clients/connect.md — default handshake called out next to the basic example
• docs/troubleshooting.md — new entry for the SdkHttpError: Error POSTing to endpoint: ... message

Test plan

• Docs review only (no runtime change)
• Confirm links between protocol-versions tip and troubleshooting heading render

Closes #2410

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