Batch path nests tools inside generationConfig, producing an invalid batch request

Environment: langextract at HEAD 9853e44, Vertex AI backend, batch mode.

Bug: GeminiLanguageModel with tools configured produces an invalid batch request. The batch branch in langextract/providers/gemini.py pops system_instruction and safety_settings out of the config to place them at the request top level, but never pops tools (allowlisted in _API_CONFIG_KEYS at gemini.py:112 and documented in the constructor as forwarded to the API). gemini_batch._build_request then camelizes the leftover key into generationConfig, producing:

{"generationConfig": {"temperature": 0.0, "tools": [{"google_search": {}}], "candidateCount": 1}}

In the GenerateContentRequest schema, tools is a top-level request field like systemInstruction and safetySettings, not a GenerationConfig field, so Vertex batch validation rejects the request. With ignore_item_errors the outputs silently pad to empty strings instead. The identical model config works on the realtime path because generate_content(config=call_config) accepts tools at that level.

Repro (no API call needed):

from langextract.providers import gemini_batch
gen_config = {"temperature": 0.0, "tools": [{"google_search": {}}], "candidate_count": 1}
req = gemini_batch._build_request(prompt="p", gen_config=gen_config, ...)
# req["generationConfig"] contains "tools"; the schema wants it at req top level

Trigger: GeminiLanguageModel(vertexai=True, project=..., location=..., tools=[{"google_search": {}}], batch={"threshold": 1}) then extract/infer over >= threshold prompts.

Fix shape: pop tools in the batch branch next to the system_instruction/safety_settings pops and pass it through infer_batch into _build_request, which sets request["tools"] at the top level, mirroring the existing systemInstruction/safetySettings handling. Every other _API_CONFIG_KEYS member left in the dict is a legitimate generationConfig field; tools is the single misplaced one.

I have this fix implemented with a regression test and can open a PR.