[v2] Add Session IDs to User-Agent header even when cache does not exist

[aemous]

Issue #, if available:

• CLI-8466 (internal)

Description of changes:

• Fixed a bug where session IDs are added to the User-Agent HTTP header even when the local cache does not exist.
• Modified CLISessionDatabaseConnection to allow a cache directory override to be passed in during initialization. This improves testability.
• Added new regression test that verifies (1) local cache is created and (2) database connection succeeds when the local cache does not exist.

Description of tests:

• Successfully ran all tests and the pre-production build workflow (see GitHub Actions).
• Ran a manual integration test workflow to verify expected behavior (manual test workflow described below).

Manual test workflow

1. Deleted ~/.aws/cli/cache: rm -rf ~/.aws/cli/cache.
2. Installed AWS CLI v2 WITHOUT the changes.
3. Ran the following command: python3 -m awscli s3api list-buckets --debug 2> debug.txt.
4. Inspected debug.txt and confirmed the User-Agent HTTP header does not contain a session ID: aws-cli/2.35.2 md/awscrt#0.32.2 ua/2.1 os/macos#25.5.0 md/arch#arm64 lang/python#3.12.6 md/pyimpl#CPython m/c,E,C,a,n cfg/retry-mode#standard md/installer#source md/prompt#off md/command#s3api.list-buckets.
5. Deleted ~/.aws/cli/cache: rm -rf ~/.aws/cli/cache.
6. Installed AWS CLI v2 WITH the changes (HEAD of branch).
7. Ran the following command: python3 -m awscli s3api list-buckets --debug 2> debug.txt.
8. Inspected debug.txt and confirmed the User-Agent HTTP header contains a session ID: aws-cli/2.35.2 md/awscrt#0.32.2 ua/2.1 os/macos#25.5.0 md/arch#arm64 lang/python#3.12.6 md/pyimpl#CPython m/n,C,E,c,a cfg/retry-mode#standard md/installer#source sid/b871d801befe md/prompt#off md/command#s3api.list-buckets

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