fix: string-to-datetime prep transform fails on timestamps without seconds

[iabaako]

Pull Request Summary 🚀

What does this PR do? 📝

Fixes #261: converting a string column to datetime via a Prep step crashed the whole Streamlit app with an uncaught traceback whenever the value's time component had no seconds (e.g. 3/15/2026 17:28).

• Adds no-seconds (H:MM/HH:MM) format variants alongside every existing date+time shape in _parse_flexible_datetime (ISO, Stata-style 18aug2025, M/D/Y, D/M/Y), so timestamps missing seconds now parse correctly.
• Wraps the interactive "Add Prep Step" apply call (prep_add_step → prep_apply_action) in try/except (ValidationError, OperationError) with st.error(...), matching the pattern already used by the correction view's interactive apply, so a still-unsupported format now shows a friendly error instead of crashing the whole app.

Why is this change needed? 🤔

Timestamps without seconds are a common real-world shape (spreadsheet/Excel exports, some submissiondate-like fields), and every format _parse_flexible_datetime tried required HH:MM:SS. On top of the parsing gap, the interactive prep-apply path had no error boundary, so any parse failure took down the whole app with a raw traceback rather than a usable message.

How was this implemented? 🛠️

• processing/prep.py: added a %H:%M-only validator/format pair immediately after each existing %H:%M:%S entry in TransformColumnsOperation._parse_flexible_datetime's formats_to_try list. The no-seconds validators are anchored ($) so they can't accidentally match a with-seconds value.
• views/prep_view.py: wrapped prep_apply_action(project_id, label, PrepActionResult(**prep_args)) in prep_add_step's Add-button handler in try/except, showing st.error(f"Error adding preparation step: {e!s}") on failure and only calling st.success/st.rerun on success.

How to test or reproduce? 🧪

1. uv run python -m pytest — full suite passes.
2. just lint-py / just fmt-python / pre-commit — clean.
3. Manual: add a "string to datetime" prep step on a column containing 3/15/2026 17:28 — it now converts successfully instead of crashing the app.
4. Manual: add a "string to datetime" prep step on a column with a genuinely unsupported format — the page now shows a red error message instead of an uncaught traceback.

Screenshots (if applicable) 📷

N/A — bug fix, no new visual components.

Checklist ✅

• I have run and tested my changes locally
• I have limit this PR to less than 1000 lines of code change (if not, explain why)
• I have updated/added tests to cover my changes (if applicable)
• I have updated/added requirements to cover my changes (if applicable) — N/A, no new dependencies
• I have run linting and formatting on any code changes (if applicable)
• I have updated the documentation (README, etc.) accordingly — N/A, no user-facing docs changes needed
• I have reviewed and resolved any merge conflict

Note on stacking

This PR is stacked on fix/253-standardize-reapply-error-handling (#260), which is still an open draft. Base is set to that branch rather than main so review only shows this PR's incremental diff; once #260 merges, retarget this PR's base to main.

🤖 Generated with Claude Code

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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