docs(mcp): add ID-JAG auth guide with connect-time credential errors

[devin-ai-integration]

Summary

Adds docs/mcp_id_jag.md (the oauth2_id_jag MCP auth mode) and links it from the auth overview, the mcp.md auth_type table and the sidebar. oauth2_id_jag has been shipping since BerriAI/litellm#31516 with no page in these docs

The page carries over the Okta setup guide from the unmerged #425 and brings it up to date with two later proxy changes. BerriAI/litellm#35147 made the leg-1 subject come from the identity assertion captured at LiteLLM SSO login when the request has no Authorization bearer, so a virtual key alone is enough; the "Calling an ID-JAG MCP Server" section now describes both subject sources and MCP_SSO_ASSERTION_CACHE_TTL_SECONDS. BerriAI/litellm#35392 (LIT-5062) pre-flights the ID-JAG credential on single-server routes before the session opens, so a missing or expired stored assertion is a 412 with the actionable body (no WWW-Authenticate, deliberately) and an unreachable assertion store is a 503, instead of a 200 with "tools": [] and a tools/call that says the tool does not exist. The new "Credential Errors at Connect Time" section documents that, the per-server _meta.litellm.ai/server_outcomes shape the aggregate /mcp route still returns, and the known bearer-carrying tools/list mismatch tracked as LIT-5067. Troubleshooting rows were added for each

Supersedes #425 if merged. npm run build passes

Resolves LIT-5062 (docs)

Link to Devin session: https://app.devin.ai/sessions/2e35eeaf6134486da2665d8d496a534e
Open in Devin Desktop: https://app.devin.ai/desktop/session/2e35eeaf6134486da2665d8d496a534e?variant=devin
Requested by: @yassin-berriai

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Project	Deployment	Actions	Updated
litellm	Ready	Preview	Sep 4, 2026 1:28am UTC