fixes #5928
fixes #7129

PiperOrigin-RevId: 974803581

PiperOrigin-RevId: 976484701

PiperOrigin-RevId: 978679098

`Sandboxes.send_command` built an inner `genai.Client(vertexai=True, ...)` and
called `_api_client.request(...)` to talk to the sandbox data plane. That genai
client's authorized session has no credentials object, so every call failed with
`AttributeError: 'NoneType' object has no attribute 'before_request'`, making all
Computer Use / sandbox HTTP calls (GET /, /tabs, POST /cdp, etc.) unusable.

The sandbox data plane does not use ADC; it authenticates via the
`Authorization: Bearer <jwt>`, `X-Sandbox-Routing-Token` and `X-Sandbox-Port`
headers that `send_command` already sets. Routing through a genai client was
therefore both unnecessary and broken.

Issue the request directly with `requests` instead, mirroring
`generate_access_token`, which already talks to a non-aiplatform endpoint the
same way. A JSON `Content-Type` is set only when a request body is present, and a
caller-supplied `Content-Type` is preserved. `generate_browser_ws_headers`,
which calls `send_command` internally, continues to parse the JSON body
correctly.

PiperOrigin-RevId: 979899791

…lease

PiperOrigin-RevId: 981243301

Co-authored-by: release-please[bot] <55107282+release-please[bot]@users.noreply.github.com>