Support Guzzle 7 and 8 without SDK API changes

[MarketDataApp]

Summary

• allow both Guzzle ^7.8 and ^8.0, keeping existing Guzzle 7 installations valid
• handle request and network failures through stable PSR-18 interfaces
• preserve response objects for response-bearing failures and preserve null for no-response network failures
• update transport fixtures for the Guzzle 8 exception hierarchy
• make no changes to public SDK constructors, methods, properties, return types, or SDK exception types

Supersedes #80.

Validation

• Guzzle 7.10: 1,082 unit tests, 3,358 assertions
• Guzzle 7.8 with lowest supported dependencies: 1,082 unit tests, 3,358 assertions
• Guzzle 8.1: 1,082 unit tests, 3,358 assertions
• Guzzle 8.1 against the live authenticated API: 20 integration tests, 6,796 assertions
• composer validate passes with clean dependency resolutions

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.78%. Comparing base (08a195a) to head (50b1cd6).

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #87   +/-   ##
=========================================
  Coverage     99.78%   99.78%           
- Complexity      969      971    +2     
=========================================
  Files            50       50           
  Lines          2823     2829    +6     
=========================================
+ Hits           2817     2823    +6     
  Misses            6        6           

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