feat(tests): auto-merge all model list responses and unify recordings (#3320)

One needed to specify record-replay related environment variables for
running integration tests. We could not use defaults because integration
tests could be run against Ollama instances which could be running
different models. For example, text vs vision tests needed separate
instances of Ollama because a single instance typically cannot serve
both of these models if you assume the standard CI worker configuration
on Github. As a result, `client.list()` as returned by the Ollama client
would be different between these runs and we'd end up overwriting
responses.

This PR "solves" it by adding a small amount of complexity -- we store
model list responses specially, keyed by the hashes of the models they
return. At replay time, we merge all of them and pretend that we have
the union of all models available.

## Test Plan

Re-recorded all the tests using `scripts/integration-tests.sh
--inference-mode record`, including the vision tests.

tests/README.md

@@ -38,26 +38,15 @@ For running integration tests, you must provide a few things:
   - a distribution name (e.g., `starter`) or a path to a `run.yaml` file
   - a comma-separated list of api=provider pairs, e.g. `inference=fireworks,safety=llama-guard,agents=meta-reference`. This is most useful for testing a single API surface.
 
-- Whether you are using replay or live mode for inference. This is specified with the LLAMA_STACK_TEST_INFERENCE_MODE environment variable. The default mode currently is "live" -- that is certainly surprising, but we will fix this soon.
-
 - Any API keys you need to use should be set in the environment, or can be passed in with the --env option.
 
 You can run the integration tests in replay mode with:
 ```bash
 # Run all tests with existing recordings
-LLAMA_STACK_TEST_INFERENCE_MODE=replay \
-  LLAMA_STACK_TEST_RECORDING_DIR=tests/integration/recordings \
   uv run --group test \
   pytest -sv tests/integration/ --stack-config=starter
 ```
 
-If you don't specify LLAMA_STACK_TEST_INFERENCE_MODE, by default it will be in "live" mode -- that is, it will make real API calls.
-
-```bash
-# Test against live APIs
-FIREWORKS_API_KEY=your_key pytest -sv tests/integration/inference --stack-config=starter
-```
-
 ### Re-recording tests
 
 #### Local Re-recording (Manual Setup Required)
@@ -66,7 +55,6 @@ If you want to re-record tests locally, you can do so with:
 
 ```bash
 LLAMA_STACK_TEST_INFERENCE_MODE=record \
-  LLAMA_STACK_TEST_RECORDING_DIR=tests/integration/recordings \
   uv run --group test \
   pytest -sv tests/integration/ --stack-config=starter -k "<appropriate test name>"
 ```