fix(pubsub): avoid max lease overflow errors

[dbolduc]

Fixes #6721

[gemini-code-assist]

Code Review

This pull request introduces a maximum lease duration clamp of 1000 years to prevent overflow errors when setting the maximum lease, and adds a corresponding unit test. The feedback suggests reducing this limit to 100 years because a 1000-year duration can still cause overflow panics when added to an Instant on platforms where Instant is represented as a 64-bit integer of nanoseconds.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.71%. Comparing base (f02b42c) to head (acefe83).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6727      +/-   ##
==========================================
- Coverage   96.72%   96.71%   -0.01%     
==========================================
  Files         313      313              
  Lines      101266   101275       +9     
==========================================
+ Hits        97948    97949       +1     
- Misses       3318     3326       +8     

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