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.

tests/test_provider_hints.py

@@ -0,0 +1,30 @@
+"""Tests for the provider import-error hint helper in interface/main.py."""
+
+from __future__ import annotations
+
+from strix.interface.main import _provider_import_hint
+
+
+def test_bedrock_boto3_hint() -> None:
+    exc = ModuleNotFoundError("No module named 'boto3'")
+    hint = _provider_import_hint(exc, "bedrock/anthropic.claude-4-5-sonnet")
+    assert hint is not None
+    assert 'pipx install "strix-agent[' in hint
+    assert "bedrock" in hint
+
+
+def test_vertex_google_hint() -> None:
+    exc = ImportError("No module named 'google'")
+    hint = _provider_import_hint(exc, "vertex_ai/gemini-3-pro-preview")
+    assert hint is not None
+    assert 'pipx install "strix-agent[' in hint
+    assert "vertex" in hint
+
+
+def test_non_import_error_returns_none() -> None:
+    assert _provider_import_hint(ConnectionError("boom"), "bedrock/whatever") is None
+
+
+def test_unrelated_provider_returns_none() -> None:
+    exc = ImportError("No module named 'something'")
+    assert _provider_import_hint(exc, "openai/gpt-4") is None