test(cli): cover api server auto create session

[YASHcode-IIITV]

Link to Issue or Description of Change

1. Link to an existing issue (if applicable):

• Related: test(cli): add coverage for adk conformance test command options #6730

Problem

The adk api_server CLI supports the --auto_create_session option, which is propagated to the FastAPI application. However, the CLI-level propagation of this option did not have dedicated regression coverage.

A future change to the CLI argument handling could cause the option to stop being forwarded correctly without being detected by the existing tests.

Solution

Added a unit test for:

adk api_server --auto_create_session

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[YASHcode-IIITV]

@wyf7107 plz review this pr