feat(cache/unstable): add peek() to TtlCache

[tomas-zijdemans]

Adding peek to TtlCache, as the only way to read without side effects bringing symmetry with LruCache.

Both caches have a "read touches state" method (get). For LRU, get promotes. For TTL with sliding expiration, get resets the timer.

... and now both have a "read without touching state" method (peek).

Note: Also fixes the bug introduced here, where the input validation was happening too late.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (008db8d) to head (be3156d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7070   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50486    50490    +4     
  Branches     8946     8949    +3     
=======================================
+ Hits        47666    47670    +4     
  Misses       2249     2249           
  Partials      571      571           

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[bartlomieju]

Thanks!