tests: Add end-to-end test for the examples

[arpitjain099]

Description of the changes being introduced by the pull request:

This adds an end-to-end test that runs the repository, uploader and client examples against each other, which is the part of #2228 that was not covered by the tuf.repository interface tests already merged in #2651.

The new tests/test_examples_e2e.py drives the three examples the way a user runs them from the command line (see their READMEs):

• examples/repository/repo is started as a live HTTP server on a free port
• examples/uploader/uploader does Trust-On-First-Use, claims a delegation, and uploads target files to that server
• examples/client/client does Trust-On-First-Use and downloads the uploaded targets

Because the client downloads through ngclient.Updater, a successful download means the metadata chain and the target hash both verified, so the test asserts the downloaded content matches what the uploader published.

It also covers the "client that has metadata version N can successfully update to N+M" case from the issue: after the client is initialized and has downloaded a first target, the uploader adds a second target (which publishes new targets, snapshot and timestamp metadata). The test checks that the snapshot version advanced and that the already-initialized client can then refresh and download the new target.

A few notes on how it stays deterministic and isolated:

• the example scripts import their private helper modules by name (from _simplerepo import SimpleRepository), so each example dir is put on PYTHONPATH
• the client and uploader store trusted metadata and keys under Path.home(), so HOME is pointed at a temporary directory; nothing touches the real home dir and every run starts from a clean repository
• readiness is handled by polling for 1.root.json (reusing tests.utils.wait_for_server for the port) rather than sleeping

The examples themselves are not modified; this only wires the existing ones together as a test. Ran locally with python -m unittest tests.test_examples_e2e (also green under the project's surrounding example tests), and ruff check / ruff format / mypy are clean on the new file.

I kept this scoped to the examples e2e slice. Whether the remaining idea in the issue (unit-testing SimpleRepository directly) is still wanted, and whether #2228 should stay open for it, is up to you.

Addresses #2228

Please verify and check that the pull request fulfills the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[jku]

I'd really like to have this -- without it the examples are likely to break.

From CI:

TimeoutError: Could not connect to the repository example on port 39201

It's been so long since I touched tests that actually setup http servers that I don't have any useful advice but that issue does reproduce locally for me.