refactor(provider/anthropic): finish fixture migration for doGenerate tests (#12511)

## background

anthropic tests were partially migrated to the fixture-based pattern -
doStream and some doGenerate tests already used
`prepareJsonFixtureResponse` / `prepareChunksFixtureResponse`, but 3
`prepareJsonResponse` helper definitions (55 total calls) still remained

## summary

- record `anthropic-text.json` and `anthropic-text.chunks.txt` fixtures
from real API (`claude-sonnet-4-5-20250929`)
- replace 23 `prepareJsonResponse({})` calls (tests that only check
request body/headers) with
`prepareJsonFixtureResponse('anthropic-text')`
- inline `server.urls` assignments for 10 main doGenerate tests that
need specific response content (reasoning, usage, citations, tool calls,
etc.)
- inline `server.urls` assignments for 8 web search tests and delete the
scoped helper
- inline `server.urls` assignments for 4 code execution tests and delete
the scoped helper
- remove the main `prepareJsonResponse` function definition and unused
`Citation`/`JSONObject` imports

Author: dancer

packages/anthropic/src/__fixtures__/anthropic-text.chunks.txt

@@ -0,0 +1,12 @@
+{"type":"message_start","message":{"model":"claude-sonnet-4-5-20250929","id":"msg_01QC4g3HwBThD4BaNtBckFDJ","type":"message","role":"assistant","content":[],"stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":12,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"cache_creation":{"ephemeral_5m_input_tokens":0,"ephemeral_1h_input_tokens":0},"output_tokens":1,"service_tier":"standard","inference_geo":"not_available"}}}
+{"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}
+{"type":"ping"}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"! I"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"'m doing well, thank you for asking"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":". How are you doing today?"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":" Is"}}
+{"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":" there anything I can help you with?"}}
+{"type":"content_block_stop","index":0}
+{"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"input_tokens":12,"cache_creation_input_tokens":0,"cache_read_input_tokens":0,"output_tokens":30}}
+{"type":"message_stop"}

packages/anthropic/src/__fixtures__/anthropic-text.json

@@ -0,0 +1,26 @@
+{
+  "model": "claude-sonnet-4-5-20250929",
+  "id": "msg_01VdEjxAP5ahtHKrrRdNBteQ",
+  "type": "message",
+  "role": "assistant",
+  "content": [
+    {
+      "type": "text",
+      "text": "Hello! I'm doing well, thanks for asking. How are you doing today? Is there anything I can help you with?"
+    }
+  ],
+  "stop_reason": "end_turn",
+  "stop_sequence": null,
+  "usage": {
+    "input_tokens": 12,
+    "cache_creation_input_tokens": 0,
+    "cache_read_input_tokens": 0,
+    "cache_creation": {
+      "ephemeral_5m_input_tokens": 0,
+      "ephemeral_1h_input_tokens": 0
+    },
+    "output_tokens": 29,
+    "service_tier": "standard",
+    "inference_geo": "not_available"
+  }
+}