fix: resolve relative links against the URL after an HTTP redirect

[devtejasx]

Related Issue(s):

• Relative links are not resolved against the URL after a redirect #1816

Description:

When a STAC object is read over HTTP and the server redirects, the requested URL became the object's self href, so relative links resolved against the old location. Per RFC 3986 section 5.1.3, the base is the URL the document was actually retrieved from.

• DefaultStacIO (urllib and urllib3 paths) and RetryStacIO record the final URL of each read via a small _set_final_href / _get_final_href pair on StacIO. The entry is cleared when a later read of the same href isn't redirected, so a stale redirect isn't reused.
• StacIO.read_stac_object and STACObject.from_file use the final URL as the self href.
• urllib3 reports the raw Location header (often relative), so it's resolved against the URL of the request that was redirected, taken from the retry history.

Custom StacIO subclasses that don't call _set_final_href keep the current behaviour.

Tests: a local HTTP server that 308-redirects /old/ to /new/, parametrized over urllib, urllib3 and RetryStacIO, and over Catalog.from_file and pystac.read_file. The new tests fail on main and pass here.

PR Checklist:

• Pre-commit hooks pass (run pre-commit run --all-files). I ran ruff check, ruff format --check and mypy on the changed files rather than the hooks; mypy reports no new errors.
• Tests pass (run pytest). tests/test_stac_io.py 26 passed. The full suite is 1051 passed, 5 skipped, with 20 shutil.Error errors in asset/item copy tests that fail the same way on main on Windows.
• Documentation has been updated to reflect changes, if applicable
• This PR maintains or improves overall codebase code coverage
• This PR's title is formatted per conventional commits

AI tool usage:

• AI tools supported the development of this PR, per the stac-utils AI tool policy. I used Claude Code to reproduce the issue, write the change and tests, and run the checks above.