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

codersdk/deployment.go

@@ -4114,6 +4114,8 @@ type AIBridgeProviderConfig struct {
 	Key string `json:"-"`
 	// BaseURL is the base URL of the upstream provider API.
 	BaseURL string `json:"base_url"`
+	// DumpDir is the directory path for dumping API requests and responses.
+	DumpDir string `json:"dump_dir,omitempty"`
 
 	// Bedrock fields (only applicable when Type == "anthropic").
 	BedrockBaseURL         string `json:"-"`