fix(auth): add Codex OAuth accounts as distinct pool entries

hermes auth add openai-codex now creates an independent
manual:device_code pool entry per account instead of routing through
the singleton _save_codex_tokens save path, which collapsed every
added account into the latest login (the second add overwrote the
first account's singleton-mirrored device_code entry). This is the
add-path half of #39236; PR #39243 (already on this branch) fixes the
re-auth half.

manual:device_code entries refresh from their own token pair
(_sync_codex_entry_from_auth_store only adopts the singleton for
source=="device_code"), so they need no providers.openai-codex
shadow. Adding the first credential marks openai-codex active (the
singleton path did this implicitly) so the setup wizard's
get_active_provider() check still passes; subsequent adds leave the
active provider untouched.

Adds SOURCE_MANUAL_DEVICE_CODE constant and a regression test that two
distinct accounts keep distinct token pairs. Updates two existing add
tests to the pool-only behavior.

Co-authored-by: glesperance <info@glesperance.com>

agent/credential_pool.py

@@ -91,6 +91,7 @@ AUTH_TYPE_OAUTH = "oauth"
 AUTH_TYPE_API_KEY = "api_key"
 
 SOURCE_MANUAL = "manual"
+SOURCE_MANUAL_DEVICE_CODE = f"{SOURCE_MANUAL}:device_code"
 
 STRATEGY_FILL_FIRST = "fill_first"
 STRATEGY_ROUND_ROBIN = "round_robin"