Validate string and scheduling model inputs

[isPANN]

Summary

• Reuse constructor validation when loading six string models and eight scheduling models.
• Rebuild derived string length bounds rather than trusting persisted caches.
• Share checks with existing create specifications and remove duplicated validation.
• Reject malformed precedence indices and deadline arrays at JSON loading boundaries; preserve integer range checks for finite schedule slots.
• Preserve public constructors, solver/reduction APIs, and JSON field names. CLI tests use shared constructor diagnostics.
• Scope: 30 files and 859 additions, based directly on main.

Verification

• Workspace all-feature unit and integration tests: 6,495 passed, 2 ignored.
• Changed-line coverage against main: 99.33% (593 of 597 instrumented lines). Four uncovered lines: three empty-alphabet error branches and one platform-width overflow error branch.
• Workspace all-target all-feature Clippy, formatting, and whitespace checks pass.

[codecov]

Codecov Report

❌ Patch coverage is 99.32886% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.14%. Comparing base (4ceac5a) to head (ab760ea).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/models/misc/closest_string.rs	96.15%	1 Missing ⚠️
src/models/misc/grouping_by_swapping.rs	95.45%	1 Missing ⚠️
...odels/misc/scheduling_with_individual_deadlines.rs	97.77%	1 Missing ⚠️
src/models/misc/string_to_string_correction.rs	96.29%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1166      +/-   ##
==========================================
+ Coverage   96.00%   96.14%   +0.13%     
==========================================
  Files        1074     1074              
  Lines      132526   133475     +949     
==========================================
+ Hits       127233   128328    +1095     
+ Misses       5293     5147     -146     

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[GiggleLiu]

Final review notes

• Pushed one mechanical commit rewriting !(!string.is_empty() || budget == 0) as string.is_empty() && budget != 0 in grouping_by_swapping.rs; clippy 1.93 denies the original under nonminimal_bool. Behaviour identical.
• Same series follow-up as Validate graph, set-system, and database deserialization #1161: consider pub fn try_new on the new constructors, and the test_ prefix on new tests.

[GiggleLiu]

Final review: cached length bounds rebuilt on load, JSON shapes preserved, rejections trace to existing asserts; make check passes locally on the merged state after the clippy rewrite. Approving.