fix(providers): declare bedrock + vertex extras and add provider import-error hints (#588)

* feat: add bedrock + vertex optional extras with install docs and import hints (#574)

Declare [project.optional-dependencies] with vertex (google-auth) and
bedrock (boto3) extras so "strix-agent[vertex]" / "strix-agent[bedrock]"
install the provider SDKs. Add an Installation section to the Bedrock docs
mirroring Vertex, and a _provider_import_hint helper in warm_up_llm that
surfaces a pip-install hint when a provider dependency is missing.

Fixes #574, #573

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* fix(providers): use pipx in install hint to match docs

A pipx-installed strix can't add an extra with 'pip install' (wrong env);
mirror the documented 'pipx install "strix-agent[...]"' command. Addresses
Greptile review.

docs/llm-providers/bedrock.mdx

@@ -3,6 +3,14 @@ title: "AWS Bedrock"
 description: "Configure Strix with models via AWS Bedrock"
 ---
 
+## Installation
+
+Bedrock requires the AWS SDK dependency. Install Strix with the bedrock extra:
+
+```bash
+pipx install "strix-agent[bedrock]"
+```
+
 ## Setup
 
 ```bash