* fix: warn when default chat provider is unset
* fix: align startup warning with provider fallback
* refactor: simplify default chat provider warning guard checks
* feat: warn when default chat provider id is invalid or missing
- Emit a warning when `default_provider_id` points to a
non-existent enabled provider, preventing silent fallback to
an unexpected model.
- Reset the warning guard before each
`provider_manager.initialize()` so configuration reloads
trigger a fresh re-evaluation.
- Harden guard checks to handle `None` `provider_settings` and
`None` provider IDs gracefully.
* test: cover fallback and invalid default provider id warnings
- Add case for `curr_provider_inst=None` to verify fallback to
`providers[0]`.
- Add case for a `default_provider_id` that does not match any enabled
provider.
* style: format default chat provider warning
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Co-authored-by: RC-CHN <1051989940@qq.com>
587286a967