Include max attempts in the amz-sdk-request header on the initial attempt

[ubaskota]

Issue #, if available:

Description of changes:
amz-sdk-request omitted max on the initial attempt because the value was only set once a response was received. Seed it at client registration so it's present on every attempt.

Testing:

• Added unit tests asserting max is present on the initial request , across retries and for service-specific overrides (DynamoDB)
• Full test suite passes

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[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.46%. Comparing base (ee748e4) to head (c9bfe1e).
⚠️ Report is 439 commits behind head on develop.
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Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3765      +/-   ##
===========================================
- Coverage    92.59%   92.46%   -0.14%     
===========================================
  Files           68       68              
  Lines        15634    15840     +206     
===========================================
+ Hits         14477    14647     +170     
- Misses        1157     1193      +36     

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[Alan4506]

LGTM. Thanks @ubaskota!