fix(code_executors): send correct field names for sandbox input files

[jordanchendev]

Problem

AgentEngineSandboxCodeExecutor was building the input-file payload with
incorrect JSON field names:

Code sent	API expects
contents	content
mimeType	mime_type

This caused all input files to be silently unreadable inside the sandbox,
producing errors such as:

pandas.errors.EmptyDataError: No columns to parse from file

Fixes #3690

Changes

• src/google/adk/code_executors/agent_engine_sandbox_code_executor.py — rename the two dict keys in the input_data['files'] list comprehension.
• tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py — add regression test test_execute_code_sends_correct_field_names_for_input_files that verifies the correct keys are sent to the API.

Testing plan

• New regression test added that asserts content and mime_type are used (was failing before the fix, passes after).
• All existing tests in the file still pass.

pytest output

uv run --extra test python -m pytest tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py -v
======================== 11 passed, 5 warnings in 2.04s ========================

pre-commit

pre-commit run --files src/google/adk/code_executors/agent_engine_sandbox_code_executor.py \
                       tests/unittests/code_executors/test_agent_engine_sandbox_code_executor.py
isort....................................................................Passed
pyink....................................................................Passed
addlicense...............................................................Passed