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

llama_stack/providers/utils/bedrock/config.py

@@ -4,53 +4,55 @@
 # This source code is licensed under the terms described in the LICENSE file in
 # the root directory of this source tree.
 
+import os
+
 from pydantic import BaseModel, Field
 
 
 class BedrockBaseConfig(BaseModel):
     aws_access_key_id: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_ACCESS_KEY_ID"),
         description="The AWS access key to use. Default use environment variable: AWS_ACCESS_KEY_ID",
     )
     aws_secret_access_key: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_SECRET_ACCESS_KEY"),
         description="The AWS secret access key to use. Default use environment variable: AWS_SECRET_ACCESS_KEY",
     )
     aws_session_token: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_SESSION_TOKEN"),
         description="The AWS session token to use. Default use environment variable: AWS_SESSION_TOKEN",
     )
     region_name: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_DEFAULT_REGION"),
         description="The default AWS Region to use, for example, us-west-1 or us-west-2."
         "Default use environment variable: AWS_DEFAULT_REGION",
     )
     profile_name: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_PROFILE"),
         description="The profile name that contains credentials to use.Default use environment variable: AWS_PROFILE",
     )
     total_max_attempts: int | None = Field(
-        default=None,
+        default_factory=lambda: int(val) if (val := os.getenv("AWS_MAX_ATTEMPTS")) else None,
         description="An integer representing the maximum number of attempts that will be made for a single request, "
         "including the initial attempt. Default use environment variable: AWS_MAX_ATTEMPTS",
     )
     retry_mode: str | None = Field(
-        default=None,
+        default_factory=lambda: os.getenv("AWS_RETRY_MODE"),
         description="A string representing the type of retries Boto3 will perform."
         "Default use environment variable: AWS_RETRY_MODE",
     )
     connect_timeout: float | None = Field(
-        default=60,
+        default_factory=lambda: float(os.getenv("AWS_CONNECT_TIMEOUT", "60")),
         description="The time in seconds till a timeout exception is thrown when attempting to make a connection. "
         "The default is 60 seconds.",
     )
     read_timeout: float | None = Field(
-        default=60,
+        default_factory=lambda: float(os.getenv("AWS_READ_TIMEOUT", "60")),
         description="The time in seconds till a timeout exception is thrown when attempting to read from a connection."
         "The default is 60 seconds.",
     )
     session_ttl: int | None = Field(
-        default=3600,
+        default_factory=lambda: int(os.getenv("AWS_SESSION_TTL", "3600")),
         description="The time in seconds till a session expires. The default is 3600 seconds (1 hour).",
     )