fix(deployment): disable fastmcp host-origin guard behind the runtime proxy

[AnayGarodia]

Fixes #4199.

Summary

FastMCP >= 3.4.3 turns on DNS-rebinding (Host header) protection by default. The MCP launcher runs behind the runtime reverse proxy (modal / tower / local runner), which rewrites the Host header, so with the allowlist defaulting to localhost the guard rejects every request with 421 Misdirected Request. The server starts but never serves a usable request.

Fix

run_mcp_instance now passes host_origin_protection to instance.run(), driven by a new McpConfiguration.host_origin_protection field that defaults to False (these servers only receive traffic from the authenticated runtime proxy, so the guard is redundant). It stays overridable for anyone who wants it on.

Because dlt pins fastmcp>=3.0.0 but the option only exists in >=3.4.3, the argument is passed only when the installed FastMCP supports it (version-gated) — passing it to an older FastMCP would raise TypeError. Older versions never had the guard, so they need no change.

This replaces the interim fastmcp<3.4.3 pin in the test workspace: real deployments on the runtime were still broken until the launcher itself was fixed.

Tests

Two unit tests in tests/workspace/deployment/test_launchers.py: the arg is passed as False when supported, and omitted when not. black / ruff / mypy pass on the changed files.

Note

I went with option 1 from the issue (disable the guard for these proxied deployments) rather than a static allowlist, since the forwarded Host differs per backend (modal *.modal.host, tower, local *.dltrun.*) so no single allowlist works. Happy to switch to a config-driven per-backend allowlist if you prefer.

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