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_optional_deps.py

@@ -0,0 +1,26 @@
+"""Tests for the optional-dependency extras declared in pyproject.toml."""
+
+from __future__ import annotations
+
+import tomllib
+from pathlib import Path
+
+
+PYPROJECT = Path(__file__).resolve().parent.parent / "pyproject.toml"
+
+
+def _optional_dependencies() -> dict[str, list[str]]:
+    data = tomllib.loads(PYPROJECT.read_text(encoding="utf-8"))
+    return data["project"]["optional-dependencies"]
+
+
+def test_vertex_extra_pins_google_auth() -> None:
+    extras = _optional_dependencies()
+    assert "vertex" in extras
+    assert any(req.startswith("google-auth") for req in extras["vertex"])
+
+
+def test_bedrock_extra_pins_boto3() -> None:
+    extras = _optional_dependencies()
+    assert "bedrock" in extras
+    assert any(req.startswith("boto3") for req in extras["bedrock"])