fix(auxiliary): resolve named custom providers and 'main' alias in auxiliary routing (#5978)

* fix(telegram): replace substring caption check with exact line-by-line match

Captions in photo bursts and media group albums were silently dropped when
a shorter caption happened to be a substring of an existing one (e.g.
"Meeting" lost inside "Meeting agenda"). Extract a shared _merge_caption
static helper that splits on "\n\n" and uses exact match with whitespace
normalisation, then use it in both _enqueue_photo_event and
_queue_media_group_event.

Adds 13 unit tests covering the fixed bug scenarios.

Cherry-picked from PR #2671 by Dilee.

* fix: extend caption substring fix to all platforms

Move _merge_caption helper from TelegramAdapter to BasePlatformAdapter
so all adapters inherit it. Fix the same substring-containment bug in:
- gateway/platforms/base.py (photo burst merging)
- gateway/run.py (priority photo follow-up merging)
- gateway/platforms/feishu.py (media batch merging)

The original fix only covered telegram.py. The same bug existed in base.py
and run.py (pure substring check) and feishu.py (list membership without
whitespace normalization).

* fix(auxiliary): resolve named custom providers and 'main' alias in auxiliary routing

Two bugs caused auxiliary tasks (vision, compression, etc.) to fail when
using named custom providers defined in config.yaml:

1. 'provider: main' was hardcoded to 'custom', which only checks legacy
   OPENAI_BASE_URL env vars. Now reads _read_main_provider() to resolve
   to the actual provider (e.g., 'custom:beans', 'openrouter', 'deepseek').

2. Named custom provider names (e.g., 'beans') fell through to
   PROVIDER_REGISTRY which doesn't know about config.yaml entries.
   Now checks _get_named_custom_provider() before the registry fallback.

Fixes both resolve_provider_client() and _normalize_vision_provider()
so the fix covers all auxiliary tasks (vision, compression, web_extract,
session_search, etc.).

Adds 13 unit tests. Reported by Laura via Discord.

---------

Co-authored-by: Dilee <uzmpsk.dilekakbas@gmail.com>

website/docs/user-guide/configuration.md

@@ -553,7 +553,7 @@ Every model slot in Hermes — auxiliary tasks, compression, fallback — uses t
 
 When `base_url` is set, Hermes ignores the provider and calls that endpoint directly (using `api_key` or `OPENAI_API_KEY` for auth). When only `provider` is set, Hermes uses that provider's built-in auth and base URL.
 
-Available providers: `auto`, `openrouter`, `nous`, `codex`, `copilot`, `anthropic`, `main`, `zai`, `kimi-coding`, `minimax`, and any provider registered in the [provider registry](/docs/reference/environment-variables).
+Available providers: `auto`, `openrouter`, `nous`, `codex`, `copilot`, `anthropic`, `main`, `zai`, `kimi-coding`, `minimax`, any provider registered in the [provider registry](/docs/reference/environment-variables), or any named custom provider from your `custom_providers` list (e.g. `provider: "beans"`).
 
 ### Full auxiliary config reference
 
@@ -704,7 +704,7 @@ auxiliary:
     model: "my-local-model"
 ```
 
-`provider: "main"` follows the same custom endpoint Hermes uses for normal chat. That endpoint can be set directly with `OPENAI_BASE_URL`, or saved once through `hermes model` and persisted in `config.yaml`.
+`provider: "main"` uses whatever provider Hermes uses for normal chat — whether that's a named custom provider (e.g. `beans`), a built-in provider like `openrouter`, or a legacy `OPENAI_BASE_URL` endpoint.
 
 :::tip
 If you use Codex OAuth as your main model provider, vision works automatically — no extra configuration needed. Codex is included in the auto-detection chain for vision.