feat(bedrock): support Amazon Bedrock API key (bearer token) authentication

[Winne004]

Describe your changes

Adds support for Amazon Bedrock API keys (bearer tokens) in the Bedrock provider path of from_provider.

Previously, _build_bedrock in instructor/v2/auto_client.py accepted api_key but silently ignored it (# noqa: ARG001), only wiring up aws_access_key_id / aws_secret_access_key / aws_session_token. Bedrock is the only major provider where api_key doesn't work, which hurts ergonomics for people trying instructor with Bedrock:

client = instructor.from_provider(
    "bedrock/anthropic.claude-3-5-sonnet-20241022-v2:0",
    api_key="your-bedrock-api-key",  # was silently ignored — now works
)

Implementation: when api_key is provided, it is exported as AWS_BEARER_TOKEN_BEDROCK before the boto3 client is created. This is the mechanism botocore (>= 1.39.0) natively supports for Bedrock bearer-token auth, and it's how other libraries in the ecosystem (e.g. langchain-aws) enable it. When no api_key is passed, behavior is unchanged — including picking up an AWS_BEARER_TOKEN_BEDROCK the user already exported themselves.

Changes:

• instructor/v2/auto_client.py — honor api_key in _build_bedrock by exporting AWS_BEARER_TOKEN_BEDROCK
• tests/v2/test_auto_client_deterministic.py — two new deterministic tests: api_key sets the bearer-token env var; no api_key leaves it unset
• docs/integrations/bedrock.md — new "Bedrock API Key (Bearer Token) Authentication" section covering both the api_key parameter and the env var, with the boto3 version requirement and credential-precedence note

Verified locally: pytest tests/v2 → 1498 passed, 182 skipped; ruff check / ruff format clean.

Issue ticket number and link

Closes #2408

Checklist before requesting a review

• I have performed a self-review of my code
• If it is a core feature, I have added thorough tests.
• If it is a core feature, I have added documentation.

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