Fix multisession assignment indexing

[FlorianPfaff]

Summary

• normalize multisession assignment session indices consistently
• compute skipped-session gaps from actual session indices instead of compact positions
• add regression coverage for non-contiguous session assignment inputs

Validation

• git diff --check
• conflict marker scan

Full test suites were not run, per request.

[github-actions]

✅MegaLinter analysis: Success

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ COPYPASTE	jscpd	yes		no	no	37.23s
✅ PYTHON	black	2	1	0	0	2.87s
✅ PYTHON	isort	2	1	0	0	0.43s
✅ REPOSITORY	checkov	yes		no	no	32.47s
✅ REPOSITORY	gitleaks	yes		no	no	17.84s
✅ REPOSITORY	git_diff	yes		no	no	0.06s
✅ REPOSITORY	secretlint	yes		no	no	13.81s
✅ REPOSITORY	syft	yes		no	no	2.81s
✅ REPOSITORY	trivy-sbom	yes		no	no	4.56s
✅ REPOSITORY	trufflehog	yes		no	no	23.68s

Notices

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[github-actions]

Test Results

    10 files  ± 0      10 suites  ±0   2h 45m 39s ⏱️ + 12m 32s
 2 166 tests + 3   2 162 ✅ + 3      4 💤 ±0  0 ❌ ±0 
22 091 runs  +30  18 300 ✅ +21  3 791 💤 +9  0 ❌ ±0 

Results for commit b0827ae. ± Comparison against base commit 220abda.