feat: add INSECURE oidc email fallback flag for IdP brokers (#26751)

<!-- Authored by Coder Agents on behalf of @Emyrk. -->

Adds an opt-in `CODER_DANGEROUS_OIDC_EMAIL_FALLBACK` flag (alias
`--dangerous-oidc-email-fallback`) for IdP brokers that do not issue a
stable `sub` for the same user across connections.

cli/testdata/server-config.yaml.golden

@@ -441,6 +441,13 @@ oidc:
   # next login.
   # (default: true, type: bool)
   oidc-repair-links: true
+  # INSECURE: Allow OIDC logins to fall back to email-based matching when the
+  # linked_id (issuer+subject) does not match an existing user link. Required for
+  # IdP brokers that do not issue a stable 'sub' for the same user across
+  # connections. The existing user_link's linked_id is preserved on fallback. Only
+  # enable if you understand and accept the risk.
+  # (default: <unset>, type: bool)
+  dangerousOidcEmailFallback: false
 # Telemetry is critical to our ability to improve Coder. We strip all personal
 #  information before sending data to our servers. Please only disable telemetry
 #  when required by your organization's security policy.