fix(coderd): send assigned chat model IDs verbatim (#28144) - #28149

Fixes #27361 (CODAGT-832).

## Problem

When an Agents model was configured under a non-gateway provider type
(e.g. Anthropic or OpenAI) with a model ID whose first `/`- or
`:`-segment matched a built-in provider name (`anthropic`, `azure`,
`bedrock`, `google`, `openai`, `openai-compat`, `openrouter`, `vercel`),
`chatprovider.ResolveModelWithProviderHint` parsed it as a canonical
`provider/model` reference: the prefix was stripped and the request
rerouted to the embedded provider type, overriding the provider the
admin explicitly assigned. LLM gateways (e.g. LiteLLM) that namespace
their catalogs as `bedrock/...` or `anthropic/...` behind an Anthropic-
or OpenAI-type provider failed with an opaque upstream "Model not
found", and escaping was impossible (`bedrock/bedrock/...` still
rerouted).

## Fix

A valid provider hint is now authoritative:
`ResolveModelWithProviderHint` returns the assigned provider and the
verbatim model ID whenever a hint is present. Canonical `provider/model`
and `provider:model` parsing applies only to hint-less resolution paths.
Every production call site derives the hint from the model config's
explicitly assigned AI provider, so the assignment always wins.

The save-time guard rejecting slash-namespaced models on OpenRouter-like
providers typed as `openai` (provider named `openrouter` or hosted at
`openrouter.ai`) is kept: that combination remains a misconfiguration
whose correct fix is the `openrouter` provider type, and rejecting it
early beats a confusing upstream error. Its wording no longer claims
prefix stripping happens.

## Back-compat note

A pre-existing config that relied on stripping (e.g. model
`anthropic/claude-x` assigned to an Anthropic-type provider pointing at
the real Anthropic API) now sends the prefixed ID verbatim and will get
a clear upstream model-not-found error; the admin fixes it by editing
the model ID. Nothing in the product ever suggested the canonical form
for assigned models.

## Validation

- Unit: `TestResolveModelWithProviderHint` updated (hints preserve
`bedrock/...`, `anthropic/...`, `provider:...` verbatim; hint-less
canonical parsing unchanged), red-green verified against the old
ordering. Gateway and openai-type provider routing tests assert verbatim
pass-through end to end.
- Full `./coderd/x/chatd/...` suites plus `TestCreateChatModelConfig`,
`TestUpdateChatModelConfig`, and
`TestValidateChatModelConfigProviderModel` pass.
- Remote dogfood UAT on real models (PASS): an openai-type provider
pointed at a Vercel AI Gateway mount returned a real completion for
`anthropic/claude-haiku-4.5`, with trace logs confirming
`provider=openai model=anthropic/claude-haiku-4.5` (verbatim, not
rerouted); gateway-type (`openai-compat`) routing with
`deepseek/deepseek-v4-pro-0813` and the model catalog/picker regressions
pass.

> Mux acted on Mike's behalf to create this PR.

(cherry picked from commit 93c6faf)