refactor(amazonq): combine IAM and SSO implementations of StreamingClientService

[liramon1]

Problem

The agentic bundle for the CodeWhisperer LSP only supports SSO. If language clients want it to consume IAM credentials, they must start a completely separate bundle which includes the IAM implementation of the CodeWhisperer LSP. This prevents clients from seamlessly switching authentication methods at runtime.

Solution

This is part of #1936.

This PR combines the IAM and SSO implementations of the StreamingClientService, which is a prerequisite for combining the IAM and SSO implementations of AmazonQServiceManager. References to StreamingClientServiceToken and StreamingClientServiceIAM will be changed in a later PR.

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 53.76884% with 184 lines in your changes missing coverage. Please review.

Project coverage is 65.86%. Comparing base (1491f4f) to head (447c8c0).

Files with missing lines	Patch %	Lines
...p-codewhisperer/src/shared/codeWhispererService.ts	37.07%	168 Missing ⚠️
...codewhisperer/src/shared/streamingClientService.ts	85.32%	16 Missing ⚠️

Additional details and impacted files

@@                    Coverage Diff                     @@
##           feature/flare-iam-base    #1959      +/-   ##
==========================================================
- Coverage                   65.96%   65.86%   -0.10%     
==========================================================
  Files                         238      238              
  Lines                       50623    50785     +162     
  Branches                     3385     3401      +16     
==========================================================
+ Hits                        33393    33452      +59     
- Misses                      17170    17273     +103     
  Partials                       60       60              

Flag	Coverage Δ
unittests	65.86% <53.76%> (-0.10%)	⬇️

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