fix: use lambda pattern for bedrock config env vars (#3307)

# What does this PR do?

Improved bedrock provider config to read from environment variables like
AWS_ACCESS_KEY_ID. Updated all
fields to use default_factory with lambda patterns like the nvidia
provider does.

  Now the environment variables work as documented.

  Closes #3305

  ## Test Plan

  Ran the new bedrock config tests:
  ```bash
python -m pytest tests/unit/providers/inference/bedrock/test_config.py
-v

Verified existing provider tests still work:
  python -m pytest tests/unit/providers/test_configs.py -v

tests/unit/providers/inference/bedrock/test_config.py

@@ -0,0 +1,63 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import os
+from unittest.mock import patch
+
+from llama_stack.providers.utils.bedrock.config import BedrockBaseConfig
+
+
+class TestBedrockBaseConfig:
+    def test_defaults_work_without_env_vars(self):
+        with patch.dict(os.environ, {}, clear=True):
+            config = BedrockBaseConfig()
+
+            # Basic creds should be None
+            assert config.aws_access_key_id is None
+            assert config.aws_secret_access_key is None
+            assert config.region_name is None
+
+            # Timeouts get defaults
+            assert config.connect_timeout == 60.0
+            assert config.read_timeout == 60.0
+            assert config.session_ttl == 3600
+
+    def test_env_vars_get_picked_up(self):
+        env_vars = {
+            "AWS_ACCESS_KEY_ID": "AKIATEST123",
+            "AWS_SECRET_ACCESS_KEY": "secret123",
+            "AWS_DEFAULT_REGION": "us-west-2",
+            "AWS_MAX_ATTEMPTS": "5",
+            "AWS_RETRY_MODE": "adaptive",
+            "AWS_CONNECT_TIMEOUT": "30",
+        }
+
+        with patch.dict(os.environ, env_vars, clear=True):
+            config = BedrockBaseConfig()
+
+            assert config.aws_access_key_id == "AKIATEST123"
+            assert config.aws_secret_access_key == "secret123"
+            assert config.region_name == "us-west-2"
+            assert config.total_max_attempts == 5
+            assert config.retry_mode == "adaptive"
+            assert config.connect_timeout == 30.0
+
+    def test_partial_env_setup(self):
+        # Just setting one timeout var
+        with patch.dict(os.environ, {"AWS_CONNECT_TIMEOUT": "120"}, clear=True):
+            config = BedrockBaseConfig()
+
+            assert config.connect_timeout == 120.0
+            assert config.read_timeout == 60.0  # still default
+            assert config.aws_access_key_id is None
+
+    def test_bad_max_attempts_breaks(self):
+        with patch.dict(os.environ, {"AWS_MAX_ATTEMPTS": "not_a_number"}, clear=True):
+            try:
+                BedrockBaseConfig()
+                raise AssertionError("Should have failed on bad int conversion")
+            except ValueError:
+                pass  # expected