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

site/src/api/typesGenerated.ts

@@ -144,6 +144,10 @@ export interface AIBridgeProviderConfig {
 	 * BaseURL is the base URL of the upstream provider API.
 	 */
 	readonly base_url: string;
+	/**
+	 * DumpDir is the directory path for dumping API requests and responses.
+	 */
+	readonly dump_dir?: string;
 	readonly bedrock_region?: string;
 	readonly bedrock_model?: string;
 	readonly bedrock_small_fast_model?: string;