* fix(core): prevent ModelSettings default max_output_tokens from overriding agent config
When a conversation's model_settings were saved, the Pydantic default
of max_output_tokens=4096 was always persisted to the DB even when the
client never specified it. On subsequent messages, this default would
overwrite the agent's max_tokens (typically None) with 4096, silently
capping output.
Two changes:
1. Use model_dump(exclude_unset=True) when persisting model_settings
to the DB so Pydantic defaults are not saved.
2. Add model_fields_set guards at all callsites that apply
_to_legacy_config_params() to skip max_tokens when it was not
explicitly provided by the caller.
Also conditionally set max_output_tokens in the OpenAI Responses API
request builder so None is not sent as null (which some models treat
as a hard 4096 cap).
* nit
* Fix model_settings serialization to preserve provider_type discriminator
Replace blanket exclude_unset=True with targeted removal of only
max_output_tokens when not explicitly set. The previous approach
stripped the provider_type field (a Literal with a default), which
broke discriminated union deserialization when reading back from DB.
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