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/llm.py

@@ -5,10 +5,10 @@ from __future__ import annotations
 import os
 from typing import Optional
 
-from pydantic import Field, field_validator
+from pydantic import Field
 
 from docsgpt.core.paths import home_dir
-from docsgpt.core.settings._shared import SettingsGroup, normalize_secret
+from docsgpt.core.settings._shared import SettingsGroup
 
 
 class LLMSettings(SettingsGroup):
@@ -104,18 +104,3 @@ class LLMSettings(SettingsGroup):
     OPENAI_REASONING_SUMMARY: str = Field(
         default="auto", description="Reasoning summary mode requested from the Responses API."
     )
-
-    @field_validator(
-        "API_KEY",
-        "OPENAI_API_KEY",
-        "ANTHROPIC_API_KEY",
-        "GOOGLE_API_KEY",
-        "GROQ_API_KEY",
-        "HUGGINGFACE_API_KEY",
-        "NOVITA_API_KEY",
-        "FALLBACK_LLM_API_KEY",
-        mode="before",
-    )
-    @classmethod
-    def _normalize_llm_secrets(cls, v):
-        return normalize_secret(v)