Add validation for custom version input in release workflow

[Copilot]

The release workflow accepted arbitrary strings for custom_version without validation, allowing invalid versions like "abc" or "1.2.3.4.5" to be written to pyproject.toml.

Changes

• Import InvalidVersion from packaging.version
• Validate custom version with parse() before accepting it
• Raise ValueError with descriptive message on invalid input

# Handle custom version
if release_type == "custom":
    # Validate custom version
    try:
        parse(custom)
        new_version = custom
    except InvalidVersion as e:
        raise ValueError(f"Invalid custom version '{custom}': {e}")

Workflow now rejects malformed versions early instead of creating invalid package metadata.

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