Speed up E2E test cases

[anna-singleton-resolver]

Use a fixture that holds open a streaming connection to speed up the e2e test cases by 4x! (on the benign_model set)

Current Speed of benign_model: ~160s
New Speed of benign_model: ~38s

We could actually go faster (all cases running in ~6s) by using subtests, but then we lose the ability to run single cases.

[anna-singleton-resolver]

#116 shoudl get merged first

[Copilot]

Pull request overview

This PR speeds up the E2E functional classification tests by reusing a single long-lived streaming gRPC connection across cases, and adds an env-var-based selector for which testcase directory to run.

Changes:

• Added StreamingSender + streaming_sender session fixture to reuse a persistent classify_images stream for E2E runs.
• Switched the E2E test to use streaming_sender.send(...) and made testcase selection configurable via ATHENA_E2E_TESTCASE_DIR.
• Documented the new E2E configuration in the README.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
tests/functional/e2e/testcases/parser.py	Adds env-var driven testcase loading (ATHENA_E2E_TESTCASE_DIR).
tests/functional/e2e/test_classify_single.py	Uses the new session-scoped streaming sender to classify each E2E case faster.
tests/functional/conftest.py	Introduces the StreamingSender helper and a session-scoped streaming_sender fixture; modifies athena_options scope and compression quality.
README.md	Documents E2E behavior and the ATHENA_E2E_TESTCASE_DIR environment variable.

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