feat(egress): add operator_credential mode to inject the shared backend credential

[doppelc]

If you register an MCP server that uses a static bearer token or API key (Backend Authentication), health checks pass, but user tool calls don't. The proxy hop strips the caller's gateway auth and injects nothing.

This was reported in #1598, where the patch was declined in favor of an explicit egress mode. This PR builds that mode. Credit to @hochbit for the original patch and for surfacing the gap.

This PR adds one new egress mode: operator_credential. An admin sets it on a server, and from then on the gateway injects that server's stored Backend Authentication credential for every caller it authorizes. It works with both bearer and api_key schemes and reuses the configured header name, exactly like PAT mode.

It's strictly opt-in: auth_scheme=bearer alone changes nothing, an admin has to set the mode per server, and the deployment needs EGRESS_AUTH_ENABLED=true. Clients never see the credential. Connect configs stop emitting the server-auth placeholder.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 96.34146% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
registry/api/egress_auth_routes.py	94.82%	2 Missing and 1 partial ⚠️

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