refactor(settings): treat unset spellings of every optional string as None

Review follow-up. The per-group secret validators normalised a hand-picked
list of API keys, which left other optional credentials and overrides
(OPEN_ROUTER_API_KEY, S3 and Daytona keys, ELASTIC_PASSWORD, the OIDC
trio, connector client ids, MICROSOFT_AUTHORITY, MCP_OAUTH_REDIRECT_URI)
holding the literal "None" or "" a .env file spells "unset" with, so
truthiness checks and fallbacks downstream saw a value. One rule on the
group base replaces those lists: every Optional[str] field maps "", "None"
and whitespace to None and strips real values. Plain str fields are left
alone. The OIDC required-settings check therefore also rejects those
spellings.

EMBEDDINGS_POOLING is Literal["cls", "mean"] with case-insensitive
parsing; its consumer silently ignored anything else.

Bounds added where the consumer rejects or misbehaves on the value:
SCHEDULE_RUN_OUTPUT_RETENTION_DAYS and MESSAGE_EVENTS_RETENTION_DAYS (the
cleanup repositories raise on <= 0), EMBEDDINGS_DELEGATE_TIMEOUT, the
remote-device idle/pairing/invocation TTLs and CELERY_VISIBILITY_TIMEOUT
(> 0), REMOTE_DEVICE_CMD_QUEUE_TTL_SECONDS (> 605, the documented drain
deadline), GRAPHRAG_MAX_CHUNKS_FOR_EXTRACTION (>= 0; negative would slice
the pending list from the end).

The generated reference now renders generic type arguments
(dict[str, int] rather than dict).

docsgpt/core/settings/scheduler.py

@@ -25,4 +25,4 @@ class SchedulerSettings(SettingsGroup):
     SCHEDULE_ONCE_MAX_HORIZON: int = Field(
         default=31_536_000, description="How far ahead a one-off run may be scheduled, in seconds (one year)."
     )
-    SCHEDULE_RUN_OUTPUT_RETENTION_DAYS: int = Field(default=90, description="Days scheduled-run output is kept.")
+    SCHEDULE_RUN_OUTPUT_RETENTION_DAYS: int = Field(default=90, gt=0, description="Days scheduled-run output is kept.")