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

coderd/apidoc/docs.go

@@ -13312,6 +13312,10 @@ const docTemplate = `{
                 "bedrock_small_fast_model": {
                     "type": "string"
                 },
+                "dump_dir": {
+                    "description": "DumpDir is the directory path for dumping API requests and responses.",
+                    "type": "string"
+                },
                 "name": {
                     "description": "Name is the unique instance identifier used for routing.\nDefaults to Type if not provided.",
                     "type": "string"

coderd/apidoc/swagger.json

@@ -11860,6 +11860,10 @@
 				"bedrock_small_fast_model": {
 					"type": "string"
 				},
+				"dump_dir": {
+					"description": "DumpDir is the directory path for dumping API requests and responses.",
+					"type": "string"
+				},
 				"name": {
 					"description": "Name is the unique instance identifier used for routing.\nDefaults to Type if not provided.",
 					"type": "string"