Fix _parse_formula to support covariates before interaction term

[SermetPekin]

Methodology references (required if estimator / math changes)

Method name(s): N/A — no estimator logic or math changed. This fixes a formula string parser, not the underlying estimation.
Paper / source link(s): N/A
Any intentional deviations from the source (and why): N/A
Validation

Tests added/updated: Added test_parse_formula.py with 6 tests covering:
Covariates before * (e.g. y ~ x1 + treated * post) — was broken, now fixed
Covariates on both sides of * (e.g. y ~ x1 + treated * post + x2) — was broken, now fixed
Multiple ~ gives clear error
Extra/no whitespace handling
Formula-vs-explicit-params equivalence (ATT and SE match to rtol=1e-10)
All 9 existing formula tests in test_methodology_did.py and test_estimators.py pass unchanged (no regressions)
Backtest / simulation / notebook evidence (if applicable): N/A
Security / privacy

Confirm no secrets/PII in this PR: Confirmed — changes are limited to formula parsing logic and test data generated with np.random.