Fix graph exporter output extension handling and clarify generic exporter text

[adulau]

Motivation

• Ensure the output file name/extension matches the selected export format so users don't end up with a .graphml file containing DOT or JSON content.
• Clarify that the script is a generic MISP galaxy graph exporter supporting GraphML, Graphviz DOT and a JSON graph format, not GraphML-only.

Description

• Updated module docstring and CLI help to describe the tool as a generic MISP galaxy graph exporter supporting graphml, dot, and json-graph formats.
• Added output_suffix() and resolve_output_path() helpers to map --output-format to the expected file suffix and adjust the requested path accordingly.
• Changed main() to write to the resolved output_path for all formats and updated the final status message to reference the resolved path.

Testing

• Ran python3 tools/gen_graphml.py --output-format dot --output /tmp/misp-galaxies.graphml and it succeeded, writing to /tmp/misp-galaxies.dot.
• Ran python3 tools/gen_graphml.py --output-format json-graph --output /tmp/misp-galaxies.graphml and it succeeded, writing to /tmp/misp-galaxies.json.
• Ran python3 tools/gen_graphml.py --output-format graphml --output /tmp/misp-galaxies.dot and it succeeded, writing to /tmp/misp-galaxies.graphml.

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Codex Task