Handle Omitted Hypthetical Solid Solutions in EQ3 Output

[dglmoore]

Summary

OutputParser3 failed to parse EQ3 (.3o) output files that omit the hypothetical solid
solutions and fugacities sections. read_product_phases raised a PARSER_ERROR when the
"Saturation States of Hypothetical Solid Solutions" header was not found before EOF, even though
both sections are optional.

These two sections are optional but always appear together, with the hypothetical solid solutions
preceding the fugacities. parse() now probes for the leading header and only reads both sections
when it is present, skipping both when it is absent. The change is scoped to OutputParser3; the
shared base-class helpers (read_product_phases, read_fugacities) are untouched.

Closes #187.

Type of Change

• Bug fix

Testing

Added test_can_parse_3o_without_hypothetical_solid_solutions, backed by a new
no-hypothetical-solid-solutions.3o fixture that omits both sections. The full parser suite passes,
including the existing fixtures that do contain the sections.

• Existing tests pass (pytest)
• New tests added to cover this change
• Manually verified against a real EQ3/6 run (if applicable)

Checklist

• Code follows the style guides (ruff check, ruff format, basedpyright)
• Pre-commit hooks pass (pre-commit run --all-files)
• Public functions and methods have type annotations and docstrings
• CHANGELOG.md updated under [Unreleased] (for user-facing changes)