chore: clean up env var usage in aibridge (#24783)

> AI tools where used when creating this PR

This PR removes environment variable parsing from `/aibridge` directory.

Added env variables/flags for dump dir as coder options.
Only added to new indexed provider options
(`CODER_AIBRIDGE_PROVIDER_<N>_*`) not to deprecated legacy env variables
(`CODER_AIBRIDGE_ANTHROPIC_*` and `CODER_AIBRIDGE_OPENAI_KEY_*`).

Reverted adding `MaxRetries` option as it will be removed soon due to
key failover work:
https://github.com/coder/coder/pull/24783#discussion_r3155544808

docs/reference/api/general.md

@@ -210,6 +210,7 @@ curl -X GET http://coder-server:8080/api/v2/deployment/config \
             "bedrock_model": "string",
             "bedrock_region": "string",
             "bedrock_small_fast_model": "string",
+            "dump_dir": "string",
             "name": "string",
             "type": "string"
           }