Improve parser diagnostics for missing ':' in equation units

[bluemoon-o2]

Summary

Improve equation parse diagnostics for model descriptions that miss : before unit declarations.

What Changed

• Improved error formatting in parse_string_equations to provide clearer line-level diagnostics.
• When parsing with ZeroOrMore(EQUATION) yields a degraded top-level error, re-parse the relevant line with EQUATION to recover specific expectations.
• Added a targeted hint for likely missing unit separators:
  • Equation syntax error: expected ':' before unit declaration.
  • Includes a concrete example (dv/dt = -v / tau : 1).
• Reduced brittle error detection by inspecting ParseException.parser_element metadata instead of relying on raw exception strings.
• Kept grammar behavior unchanged; this PR only improves diagnostics.

Edge-Case Handling

• Preserves specific non-unit errors (e.g. dv/d = ... still reports Expected 'dt').
• Avoids missing-: hints for malformed RHS expression cases (e.g. x = ().
• Covers both equation form and parameter form missing separators (e.g. tau second).

Tests

Added test_parse_error_messages coverage for:

• Missing : in equation definitions.
• Missing : in multiline/comment inputs.
• dt typo should not be masked by missing-unit hints.
• Malformed RHS should not trigger missing-unit hints.
• Missing : in parameter declarations.
• Unknown-unit-like token after expression (... foobaz) should still surface missing-unit hint.
• Non-equation junk input should not trigger missing-unit hints.

Validation run:

• pytest -q brian2/tests/test_equations.py
• Result: 15 passed, 1 skipped

Refs #1461

[bluemoon-o2]

For context, this PR overlaps with #1787 on improving missing-unit diagnostics, but intentionally keeps a narrower scope.

Compared to #1787, this PR focuses on a minimal fix path for #1461:

• clearer diagnostics for missing : before units,
• preservation of specific non-unit errors,
• targeted regression tests.

It does not include broader parser refactors or new equation metadata/features. If preferred, I can follow up with separate PRs for additional diagnostics after this lands.

[mstimberg]

Hi @bluemoon-o2. Could you please use the pre-commit tool to fix the formatting of the file (as described here: https://brian2.readthedocs.io/en/stable/developer/guidelines/style.html#code-style) so that our linter is happy and the test suite can run? Thanks!

[bluemoon-o2]

Thanks for the reminder! I ran pre-commit on all files and pushed the formatting fix in commit a2d6a41.

I also re-ran the relevant tests (...............s [100%]
=============================== warnings summary ===============================
../../miniconda3/lib/python3.13/site-packages/_pytest/config/init.py:1428
/home/bluemoon/miniconda3/lib/python3.13/site-packages/_pytest/config/init.py:1428: PytestConfigWarning: Unknown config option: timeout

self._warn_or_fail_if_strict(f"Unknown config option: {key}\n")

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
15 passed, 1 skipped, 1 warning in 0.69s), which pass (15 passed, 1 skipped).