fix(core): Decouple provider prefix from model name in init_chat_mode… (#34046)

:…l logic

Addresses Issue #34007.
Fixes a bug where aliases like 'mistral:' were inferred correctly as a
provider but the prefix was not stripped from the model name, causing
API 400 errors. Added logic to strip prefix when inference succeeds.

**Description**
This PR resolves a logic error in `init_chat_model` where inferred
provider aliases (specifically `mistral:`) were correctly identified but
not stripped from the model string.

**The Problem**
When passing a string like `mistral:ministral-8b-latest`, the factory
logic correctly inferred the provider as `mistralai` but failed to enter
the string-splitting block because the alias `mistral` was not in the
hardcoded `_SUPPORTED_PROVIDERS` list. This caused the raw string
`mistral:ministral-8b-latest` to be passed to the `ChatMistralAI`
constructor, resulting in a 400 API error.

**The Fix**
I updated `_parse_model` in
`libs/langchain/langchain/chat_models/base.py`. The logic now attempts
to infer the provider from the prefix *before* determining whether to
split the string. This ensures that valid aliases trigger the stripping
logic, passing only the clean `model_name` to the integration class.

**Issue**
Fixes #34007

**Dependencies**
None.

**Verification**
Validated locally with a reproduction script:
- Input: `mistral:ministral-8b-latest`
- Result: Successfully instantiates `ChatMistralAI` with
`model="ministral-8b-latest"`.
- Validated that standard inputs (e.g., `gpt-4o`) remain unaffected.

Co-authored-by: ioop <[email protected]>