Require closable async iterators

[elimoss]

In a few places we are passing around async iterators that may or may not implement aclose. tighten this to specify that they do and add unconditional call to aclose after exit or cancellation. this allows more reliable stream interruption handling and lifecycle management in preparation for #471

[Copilot]

Pull request overview

This PR tightens streaming interfaces in lmi to require closable async generators and adds explicit aclose() calls so upstream provider streams can be interrupted and cleaned up reliably (in preparation for PR #471’s streaming behavior).

Changes:

• Narrow streaming type contracts from generic AsyncIterable/AsyncIterator to AsyncGenerator where closure is required.
• Ensure committed/limited streaming wrappers close their upstream generators via aclose().
• Add tests asserting upstream streams are closed when wrappers are closed.

Reviewed changes

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

File	Description
packages/lmi/src/lmi/llms.py	Tightens streaming return types to AsyncGenerator and adds upstream aclose() in committed and limiter wrappers.
packages/lmi/src/lmi/cost_tracker.py	Adds TrackedStreamWrapper.aclose() delegating to the wrapped provider stream.
packages/lmi/tests/test_dispatch.py	Updates generator typing and adds a regression test ensuring committed streams close their source.
packages/lmi/tests/test_cost_tracking.py	Adds a test ensuring TrackedStreamWrapper.aclose() closes the wrapped stream.

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