refactor(mcp): relocate the v2 gateway to litellm/proxy/gateway/mcp

Moves the clean-room gateway package out of litellm/proxy/_experimental/mcp_server/v2
into its final home litellm/proxy/gateway/mcp (the folder graduation the migration plan
had slated for Phase 2, pulled forward so all further work lands at the final path with
no later import churn). Sitting outside _experimental/mcp_server also sharpens the
clean-room boundary: the gateway no longer nests under the v1 MCP code it must not import.

Pure relocation, no behavior change: git-tracked renames preserve history; relative imports
are unaffected; the test's absolute imports, the README's basedpyright command, and the
pyrightconfig venvPath depth are updated for the new location. Tests move to
tests/mcp_tests/gateway. black, ruff, basedpyright strict, and the 16 resolver tests all
pass at the new path.

litellm/proxy/gateway/__init__.py

@@ -0,0 +1,5 @@
+"""LiteLLM MCP Gateway — the clean-room v2 gateway, grafted into v1 behind a flag.
+
+Written v2-native; nothing here imports from v1 (`litellm.proxy._experimental.mcp_server.*`).
+v1 only ever reaches this package through a thin adapter, never the other way round.
+"""

litellm/proxy/gateway/mcp/README.md

@@ -41,7 +41,7 @@ error. That is the property we want, so the gate is load-bearing rather than dec
 Run the gate (passes clean, 0 errors):
 
 ```
-basedpyright --project litellm/proxy/_experimental/mcp_server/v2/pyrightconfig.json
+basedpyright --project litellm/proxy/gateway/mcp/pyrightconfig.json
 ```
 
 Now prove the gate actually bites. Delete any one arm from `label_enum` in

litellm/proxy/gateway/mcp/pyrightconfig.json

@@ -2,7 +2,7 @@
     "include": ["."],
     "typeCheckingMode": "strict",
     "pythonVersion": "3.10",
-    "venvPath": "../../../../..",
+    "venvPath": "../../../..",
     "venv": ".venv",
     "reportMissingTypeStubs": false,
     "reportUnknownMemberType": true,

tests/mcp_tests/gateway/test_upstream_credentials.py

@@ -8,11 +8,11 @@ import httpx
 import pytest
 from pydantic import ValidationError
 
-from litellm.proxy._experimental.mcp_server.v2.oauth.httpx_auth import (
+from litellm.proxy.gateway.mcp.oauth.httpx_auth import (
     NoOpAuth,
     StaticHeaderAuth,
 )
-from litellm.proxy._experimental.mcp_server.v2.oauth.types import (
+from litellm.proxy.gateway.mcp.oauth.types import (
     ApiKeyConfig,
     AuthSpecKind,
     NoneConfig,
@@ -20,10 +20,10 @@ from litellm.proxy._experimental.mcp_server.v2.oauth.types import (
     ServerSpec,
     Subject,
 )
-from litellm.proxy._experimental.mcp_server.v2.oauth.upstream_credentials import (
+from litellm.proxy.gateway.mcp.oauth.upstream_credentials import (
     UpstreamCredentialProvider,
 )
-from litellm.proxy._experimental.mcp_server.v2.result import Error, Ok
+from litellm.proxy.gateway.mcp.result import Error, Ok
 
 PROVIDER = UpstreamCredentialProvider()
 SUBJECT = Subject(tenant_id="t1", subject_id="u1")