fix: handle session cookies with no expiry

[veronika-alraheem]

PR summary

When using session cookies there is no expires or max-age attribute available. This raises a TypeError when we try to compare the None to the current time.

Fixes i1361

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

When authenticating with a session cookie, the AuthSession cookie has no Expires or Max-Age attribute. The requests library parses cookie.expires as None in this case. This raised a TypeError as soon as ibm_cloud_sdk_core's TokenManager compared the None expire_time against the current time.

What is the new behavior?

It now falls back to a default TTL of 400 days when cookie.expires is None.

Does this PR introduce a breaking change?

• Yes
• No

Other information

Added a regression test that simulates a session cookie with no Expires/Max-Age attribute and verifies that authentication succeeds without error.