docs: add OpenID connect variables

[maxesse]

Summary

Adds documentation for OpenID Connect and Microsoft Graph token placeholder support in LibreChat, covering features from PRs #9931 and #10867.

Changes

pages/docs/features/mcp.mdx

• Added new "OpenID Connect & Microsoft Graph Token Support" section under LibreChat-Specific Features
• Documents available token placeholders with guidance on when to use each
• Includes Microsoft 365 MCP server configuration example

pages/docs/configuration/librechat_yaml/object_structure/custom_endpoint.mdx

• Added "OpenID Connect Token Placeholders" section after existing placeholder tables
• Documents all 6 OIDC/Graph placeholders with descriptions
• Includes example configuration for custom endpoints

pages/docs/configuration/librechat_yaml/object_structure/mcp_servers.mdx

• Added matching "OpenID Connect Token Placeholders" section for consistency

New Placeholders Documented

Placeholder	Description
{{LIBRECHAT_OPENID_TOKEN}}	Raw OIDC access token (backend performs OBO flow)
{{LIBRECHAT_OPENID_ID_TOKEN}}	ID token for identity verification
{{LIBRECHAT_OPENID_USER_ID}}	User ID from OIDC token
{{LIBRECHAT_OPENID_USER_EMAIL}}	User email from OIDC token
{{LIBRECHAT_OPENID_USER_NAME}}	User name from OIDC token
{{LIBRECHAT_GRAPH_ACCESS_TOKEN}}	Microsoft Graph API token (LibreChat performs OBO flow)

Related PRs

• #9931 - OpenID Connect federated provider token support
• #10867 - Microsoft Graph token support with automatic OBO exchange

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