Resize partial evaluation histories safely

[OskarEichler]

Summary

Shrink evaluation history from the entries actually present rather than slicing with the configured maximum as an array index.

Reproduction

When a history configured for 100 entries currently contains only three, resizing it to two slices from index 98 and replaces the contents with nil. The candidate retains the last two entries and does nothing when the populated history is already within the new limit.

Verification

• focused partial-history grow/shrink model
• current and cumulative candidate suites: 402 tests / 2,488 assertions, zero failures/errors and three omissions
• all 111 Ruby files pass syntax
• RuboCop checks 113 files with no offenses
• RDoc, candidate packaging and Rails 8.1.3.1 loading pass

Compatibility

Unlimited and non-shrinking sizes remain unchanged; partial histories now follow the configured limit safely. Prepared with AI-assisted source review; no repository tests were changed.

[tompng]

Looks good 👍
Can you add a test for this?
Adding a test that does the below in test_eval_history.rb would be enough

out, err = execute_lines(
  "IRB.CurrentContext.eval_history = 2",
  "__",
  conf: { EVAL_HISTORY: 5 }
)