fix(tui_rpc): stop a typed model id from keeping the wrong provider pinned

[0xKT]

Summary

Selecting anything from the model picker pins its provider, so by the time someone
types /model <name> there is almost always one pinned. A bare id matching no
provider's keywords is what a vendor Raven holds no spec for looks like, and the
pin was kept for it -- sending that provider's key to a different vendor, the
mis-routing the prefix rules exist to prevent. The prefixed form of the same id was
already handed back to auto-detection for exactly this reason; the bare form fell
through.

Refusing every such id would break the two cases where the pin was right: a user
who configured a spec-less vendor and typed one of its model names, and a local
deployment, whose server names whatever models it likes and holds no key to
mis-route. So the pin is asked to justify itself instead. Its own curated list
counts as evidence -- that is where a hand-added id sits, unprefixed -- and so does
the catalogue; prefixes are stripped before comparing, which covers every spelling
the sources use. With no evidence either way the switch is refused, naming the
qualified form to type, rather than a vendor being picked on the user's behalf.

An explicit provider is never second-guessed, so the picker's own path is
untouched.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

uv run --all-extras pytest tests/ -q -p no:randomly
  5127 passed, 30 skipped, 13 deselected

uv run ruff check raven/ tests/    All checks passed
bash .claude/scripts/preflight_ci.sh   exit 0

Behaviour, driven through config.set end to end:

Typed	Pinned	Result
mistral-large-latest	openai	refused, config untouched
mistral-large-latest	mistral	pin kept (the catalogue lists it)
some-private-tune	mistral, listed in its models	pin kept
some-local-build	ollama_chat	pin kept
mistral/mistral-large-latest	openai	auto, unchanged
gpt-4o	mistral	openai, unchanged
any, with provider sent	openai	the sent provider, gate not reached

Each test was checked by breaking the code it covers and confirming it fails. One of
those runs found that the evidence check had two branches where one sufficed --
stripping the prefix already covers the spelling the second compared exactly -- so
the redundant branch is deleted rather than pinned.

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Risk

/model <bare name> can now be refused where it previously switched. It is refused
only when nothing indicates the pinned provider serves that model, which is the
case where the switch silently used the wrong credentials; the message names the
qualified form to type instead. Typing a prefixed id, or picking from the picker,
behaves as before.

Rollback is the revert of this branch.

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

Related Issues

N/A