[lyelgate]

📝 Description

What does this PR do and why is this change necessary?

Fixes #894.

linode-cli obj signurl passed any expiry straight to generate_presigned_url. A URL signed with Signature Version 4, the client's default, can be valid for at most 604800 seconds (7 days), so a longer expiry printed a URL that only returns Access Denied. That matches the report: +31536000 fails while a 24-hour expiry works.

This PR validates the expiry before signing, for both forms (epoch time and +offset), and exits with ExitCodes.ARGUMENT_ERROR and a message on stderr when it is:

• more than 7 days away: The expiry is 31536000 seconds from now, but a signed URL can be valid for at most 604800 seconds (7 days)
• not in the future (+0, or an epoch time in the past): The expiry must be in the future
• not an integer (e.g. +an-hour): Invalid expiry '+an-hour': expected an epoch time or an offset like +3600, in seconds. Before, this raised a ValueError traceback.

Other changes:

• The limit is a MAX_SIGNED_URL_EXPIRY constant in obj/config.py, next to the plugin's other constants.
• Validation runs before get_client(). Creating the client can create Object Storage keys for the user, so an invalid expiry now fails without touching their account.
• The EXPIRY help text now gives the unit (seconds) and the 7-day maximum, which resolves the TODO next to it.

✔️ How to Test

What are the steps to reproduce the issue or verify the changes?

linode-cli obj signurl <bucket> <object> +31536000   # now an error instead of a URL that returns Access Denied
linode-cli obj signurl <bucket> <object> +604800     # still prints a URL, valid for 7 days
linode-cli obj signurl <bucket> <object> +3600       # unchanged

How do I run the relevant unit/integration tests?

make test-unit
# or just the new tests:
LINODE_CLI_TEST_MODE=1 pytest tests/unit/test_plugin_obj.py -k generate_url

The new tests in tests/unit/test_plugin_obj.py mock the S3 client, so no account is needed. They cover a valid offset, the exact 7-day boundary, a valid epoch time, and seven rejected values (over the limit by offset and by epoch time, +0, a past epoch time, and non-numeric input). The rejected cases also check that the client is never created. The rejection cases fail without this change. black, isort, autoflake and pylint pass.