Addresses the hermes-sweeper review on #53765. The in-place /model switch
helper (_apply_switched_provider_request_overrides) derived a custom
provider's extra_body by provider *name* only, while build-time matching in
agent_init._merge_custom_provider_extra_body matches by provider key, base_url,
AND model. So a different model selected at the same named endpoint could
inherit an extra_body configured for another model.
Reuse the shared agent_init._custom_provider_extra_body_for_agent matcher
(provider key + base_url + model), sourcing custom_providers from the
init-time agent._custom_providers cache (fresh-load fallback if absent). A
stale extra_body is always cleared when no entry matches; non-provider
overrides (service_tier / speed from /fast) are preserved.
Tests: add nonmatching-model and endpoint-mismatch regressions; update the
existing switch tests onto the model/base_url-aware matcher.
Third in the series. The gateway rebuild path (previous two commits)
carries a custom provider's `request_overrides` (`extra_body`, e.g.
`chat_template_kwargs`) into the agent, but the *in-place* live switch used
by the TUI dashboard and the CLI — `agent.switch_model()` ->
`agent_runtime_helpers.switch_model()` — swapped
model/provider/base_url/api_key without ever updating `request_overrides`.
So a `/model` switch to a thinking-enabled custom provider in the TUI/CLI
kept the previous provider's `extra_body`.
`switch_model()` now re-derives the switched-to provider's
`request_overrides` (via `_get_named_custom_provider`) and applies it in
place, preserving non-provider overrides (`service_tier`/`speed` from
`/fast`). Logic factored into `_apply_switched_provider_request_overrides`
for testability.
Adds tests/agent/test_switch_model_request_overrides.py.
Jack