[core] fix: stop fabricating rollout log probabilities

[zackcxb]

What does this PR do?

Stop emitting fabricated zero rollout log probabilities when a trajectory does not provide them. VERL uses the presence of rollout_log_probs to enable rollout-logprob paths, so synthetic zeros can silently contaminate bypass or off-policy importance-sampling behavior.

This PR also removes the dead gateway-level base_sampling_params config layer. Session sampling parameters are already the runtime source of defaults, and merging an always-empty base dictionary is a no-op.

Checklist Before Starting

• Searched for similar PRs and issues; relevant links are listed above.
• Formatted the PR title as [core] fix: stop fabricating rollout log probabilities.

Test

python -m pytest \
  tests/uni_agent/gateway/test_gateway_actor_on_cpu.py \
  tests/uni_agent/framework/test_generate_sequences_on_cpu.py -q
# 61 passed, 4 warnings

pre-commit run --all-files --show-diff-on-failure --color=always
# ruff, ruff format, mypy, and compile-all passed

No recipe rollout was run; this focused core change is covered by the affected CPU unit tests.

API and Usage Example

GatewayActorConfig.base_sampling_params is removed. Put sampling defaults on each session instead:

config = GatewayActorConfig(tokenizer=tokenizer)
await gateway.create_session(
    "session-id",
    sampling_params={"temperature": 0.7, "top_p": 0.9},
)

Design & Code Changes

• Emit rollout_log_probs only when trajectory.response_logprobs is present, matching the adjacent conditional routed_experts emission pattern.
• Remove GatewayActorConfig.base_sampling_params and _GatewayActor._base_sampling_params.
• Pass a copy of session.sampling_params directly to the OpenAI and Anthropic adapters; their existing base_sampling_params argument names remain unchanged.
• Update Gateway tests to provide defaults through create_session(..., sampling_params=...).
• Update the framework regression test to assert that missing rollout log probabilities omit the key while reward scores remain zero-filled.

Checklist Before Submitting

• Read the Contribute Guide.
• Ran pre-commit run --all-files --show-diff-on-failure --color=always.
• Confirmed no repository docs or examples reference the removed field; included the migration example above.
• Updated the affected CPU tests.
• Confirmed the PR title matches the required format.
• Replaced all template placeholder text with real content.

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