feat(cache/unstable): add sliding expiration to TtlCache

[tomas-zijdemans]

• Sliding expiration: entries can now stay alive as long as they're being accessed, with an optional hard deadline. Useful for sessions or rate-limit windows.
• Per-entry TTL overrides via an options object (replaces the old positional parameter).
• Constructor and set() now validate inputs, rejecting bad values with clear errors.
• onEject callback is now optional, and clear() calls it for every entry even if one throws.
• delete() skips unnecessary work for non-existent keys.
• Minor clearTimeout guard to avoid redundant calls.
• Test coverage improved.

[codecov]

Codecov Report

❌ Patch coverage is 98.79518% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 94.02%. Comparing base (6df7f21) to head (e8757e8).

Files with missing lines	Patch %	Lines
cache/ttl_cache.ts	98.79%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7046   +/-   ##
=======================================
  Coverage   94.01%   94.02%           
=======================================
  Files         627      627           
  Lines       50029    50101   +72     
  Branches     8807     8830   +23     
=======================================
+ Hits        47037    47109   +72     
  Misses       2413     2413           
  Partials      579      579           

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