fix(tools): strip NetCDF tokens; anchor list_available_output_files to parquet

[romer8]

Summary

• _tool_descriptions.py: rewrite QUERY_FILES_BY_SELECTOR_DESCRIPTION with Parquet only. hoisted next to the opener; drop unsupported_format, _excluded_netcdf_count, and mixed-format sentences. Add LIST_AVAILABLE_OUTPUT_FILES_DESCRIPTION ending with Parquet only.
• tools.py:296 routes through the new constant; tools.py:444 drops "NetCDF files do not consume index slots" from the index Field description.
• test_tool_descriptions.py: drop the positive lockstep on unsupported_format and _excluded_netcdf_count (they locked the bug in); add negative "netcdf" not in desc.lower() on query_files_by_selector and a new test_list_available_output_files_description_anchor.

Why

A weak Q4 quantized model (SimonPu/GLM-4.7-Flash:Q4_K_M) replied to the user's prompt list the last 10 available dates for model cfe_nom with a hallucinated query_data tool and the prose:

For NetCDF files: Use list_available_output_files to find files, then I can help you access them.

That's factually wrong — list_available_output_files is parquet-only since v0.5.0. The cause is description-token leakage. query_files_by_selector mentioned NetCDF five times across _tool_descriptions.py and tools.py:444 as negative constraints (unsupported_format, _excluded_netcdf_count, "do not consume index slots"). Weak quantized models token-match on the format name regardless of polarity, so "NetCDF unsupported" reads as "supports NetCDF." Combined with list_available_output_files having no parquet anchor on a generically-named tool, the model assigned the NetCDF role to that tool and surfaced it to the user.

Runtime envelope keys (unsupported_format:, _excluded_netcdf_count) keep emitting on the wire — we just stop teaching the LLM their names from description prose.

Generalizes the existing "no concrete example values" convention: no format names of formats we don't support either, even as negative constraints. The rationale is added to the _tool_descriptions.py module docstring.

Test plan

• .venv-mcp/bin/python -m pytest test_mcp/ -q — 175 passed locally
• Defense-in-depth: grep -niE "netcdf|nc4|\.nc[ ,\"']" nextgen_mcp/{prompts,tools,_tool_descriptions}.py — only the module-docstring intentional reference remains
• Manual smoke against the same prompt + a weak local model — confirm the reply no longer surfaces NetCDF and list_available_dates is chosen (model-fitness dependent; this PR does not promise it for sub-7B Q4 models)