feat: Adds configurable backoff to run_and_wait

[EspenAlbert]

Summary

Add exponential backoff with jitter between retry attempts. Previously retries fired immediately with no delay.

Three new fields on ShellConfig (mirrored as kwargs on run/run_and_wait):

• retry_initial_wait (default 5s) -- base delay before first retry
• retry_max_wait (default 60s) -- cap on computed wait
• retry_jitter (default 5s) -- random [0, jitter) added to each wait

Formula: min(initial * 2^(attempt-2), max_wait) + uniform(0, jitter)

Reviewer notes

• Backward compatible: defaults produce a 5s initial wait on retry. Existing callers with attempts=1 (the default) are unaffected since backoff only triggers on attempt > 1.
• Blocking sleep: time.sleep blocks the executor thread. This is acceptable because each run already owns a dedicated thread.
• The sleep happens before ShellRunRetryAttempt is queued, so callbacks see the event only after the wait completes.

Test plan

• Existing test_check_all_config_args_in_signatures enforces the new kwargs appear on run/run_and_wait
• test_backoff_wait_values -- mocks time.sleep, verifies exponential growth [2, 4] with jitter=0
• test_backoff_respects_max_wait -- verifies cap at max_wait over 5 attempts: [2, 4, 5, 5]
• pytest-example doc at docs/examples/shell/backoff.md runs a real retry script with wall-clock assertions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.84%. Comparing base (96b84c3) to head (c89ae93).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #9      +/-   ##
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+ Coverage   82.75%   82.84%   +0.09%     
==========================================
  Files          20       20              
  Lines        1815     1825      +10     
==========================================
+ Hits         1502     1512      +10     
  Misses        313      313              

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