Bump `starlette` in v1 to fix CVE-2026-48710

[lucasoshiro]

🔴 Required Information

Currently, ADK v1 requires starlette prior to version 1, which contains the vulnerability reported in CVE-2026-48710. It was already fixed in v2 (#5894) but not in v1.

Describe the Bug:
ADK v1 depends on an outdated version of starlette that needs to be bumped to address CVE-2026-48710.

[surajksharma07]

/adk-issue-analyze