Improve String validation

[GDYendell]

Summary by CodeRabbit

• New Features

  • String fields with configured maximum length now automatically truncate longer inputs to fit
  • String length validation enforces minimum length requirements at initialization

• Tests

  • Added tests for string truncation and length validation behaviors

[coderabbitai]

📝 Walkthrough

Walkthrough

String datatype now enforces minimum length validation and provides automatic truncation to configured maximum length. The EPICS transport layer updated to handle None length values by falling back to a default maximum. Tests added to verify truncation behavior across both layers.

Changes

Cohort / File(s)	Summary
String Datatype Validation src/fastcs/datatypes/string.py	Added __post_init__ runtime validation to enforce length >= 1. Introduced validate() method that truncates values to configured maximum length.
EPICS Transport Adaptation src/fastcs/transports/epics/ca/util.py	Modified cast_to_epics_type to handle None length gracefully with fallback to DEFAULT_STRING_WAVEFORM_LENGTH for String datatype casting.
Validation Tests tests/test_datatypes.py	Added tests verifying String truncation (length=10 truncates to 10 chars), no-length passthrough, and ValueError on zero-length constraint.
EPICS Casting Tests tests/transports/epics/ca/test_ca_util.py	Updated test expectations for EPICS string truncation: inputs exceeding 256 characters now truncated to 256, and explicit length constraints applied in casting.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Poem

🐰 A string with limits, now so fine,
Truncates with grace to length's line,
No None shall pass without a care,
Defaults step in, they're always there,
Validation springs with vigor bright! 🌟

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 28.57% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Improve String validation' clearly summarizes the main change: adding validation logic to the String datatype class via new post_init and validate methods.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch string-validation

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.82%. Comparing base (a9db2b6) to head (a6fe650).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #317      +/-   ##
==========================================
+ Coverage   90.79%   90.82%   +0.02%     
==========================================
  Files          70       70              
  Lines        2543     2551       +8     
==========================================
+ Hits         2309     2317       +8     
  Misses        234      234              

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