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Send current MCP protocol version in readiness probe (#5940)

Send current protocol version in readiness probe

The readiness probe hardcoded protocolVersion "2024-11-05" in its
initialize request. MCP servers that reject that ancient revision report
as never-ready even when healthy, so ToolHive waits out the full timeout
before failing to start them.

Advertise a pinned, current revision (2025-11-25) in the initialize body
instead. The version is deliberately pinned rather than tracking
mcp.LATEST_PROTOCOL_VERSION: the upcoming 2026-07-28 revision removes the
initialize method entirely (SEP-2575), so coupling the probe to a moving
"latest" constant would eventually emit a method:"initialize" request
stamped with a version that no longer defines it. The 2026-07-28 path
(server/discover) is deferred to #5754.

Also stop sending the MCP-Protocol-Version header on the initialize
request. The spec scopes that header to post-initialization requests
carrying the negotiated version and requires a server to reject an
unsupported value with HTTP 400 -- so a header would let a backend that
only supports an older revision fail the probe, whereas body-based
version negotiation degrades gracefully to HTTP 200. This closes item 1
of #5764.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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