Revert "[STG-1573] Add providerOptions for extensible model auth (#1822)" (#1987)

## Summary

- Reverts PR #1822 which added `providerOptions` for extensible model
auth (Bedrock/Vertex)
- Removes `providerOptions` bag from model config schema, Bedrock
examples, provider config, and server-side `modelClientOptions`
storage/passthrough

## Note

- One test from PR #1980 (`accepts verified Browserbase session
settings` in `model-config-schema.test.ts`) was removed as part of this
revert since the test file was originally created by #1822. That test
may need to be re-added in a separate file.


Made with [Cursor](https://cursor.com)

<!-- This is an auto-generated description by cubic. -->
---
## Summary by cubic
Reverts the `providerOptions`-based model auth from #1822 (STG-1573) and
restores the previous auth flow: no provider options passthrough; model
auth is via the `x-model-api-key` header.

- **Refactors**
- Removed `providerOptions` and server-side `modelClientOptions`
passthrough from core and server.
- Deleted Bedrock examples and provider config; pruned related schemas
and tests.
- Simplified OpenAPI: removed ProviderOptions/ModelClientOptions
components and helpers extracting model config from request bodies.

- **Migration**
  - Remove `providerOptions` from all model configs and requests.
- Do not send `modelClientOptions` in session start; the server ignores
it.
- Send model API keys via the `x-model-api-key` header; remove any
`skipApiKeyFallback` usage.
  - If you adopted `vertex/*` names from #1822, switch to `google/*`.

<sup>Written for commit 1a3bbcb.
Summary will update on new commits. <a
href="https://cubic.dev/pr/browserbase/stagehand/pull/1987">Review in
cubic</a></sup>

<!-- End of auto-generated description by cubic. -->

Author: tkattkat