fix(gateway): preserve media dedup after streamed replies (salvage #72321)

[teknium1]

Summary

Salvages PR #72321 by @TheEpTic: every streamed gateway reply crashed with NameError: name '_history_media_paths' is not defined after the model request already succeeded — the post-stream MEDIA-delivery branch in _handle_message_with_agent referenced a local that only exists inside _run_agent_inner (introduced in 83bba79). Users got the generic platform error on ordinary text replies as well as MEDIA-bearing ones.

Root cause: cross-function scope leak — the dedup set was bound in a different function's scope than the call site reading it (AST-verified: zero direct-scope assignments of _history_media_paths in _handle_message_with_agent).

Changes

• gateway/run.py: derive the dedup set from the handler's own loaded transcript via _collect_history_media_paths(history) — same semantics as the original dedup contract, no empty-set fallback.
• tests/gateway/test_42039_duplicate_user_message.py: regression test exercising the real already_sent branch, asserting prior-turn MEDIA paths reach _deliver_media_from_response.

Validation

	Before (main)	After
Streamed reply, already_sent=True	NameError, generic platform error	delivered normally
Regression test (sabotage run against main's line)	FAILED with the exact NameError	passes
scripts/run_tests.sh tests/gateway/test_42039_duplicate_user_message.py	4 passed / 1 failed	5 passed

Two independent production confirmations on the original PR thread (Fedora + macOS Telegram gateways) verified both the crash on current main and this fix.

Duplicate cluster: #72228 (@Ieqzya, earliest report — empty-set fallback drops dedup + unrelated desktop files), #72317 (@jorgeblanc9, same one-liner, no test), #72326 (@kylesteele-sudo, valid alternative placement + test). This salvage uses #72321's implementation; @TheEpTic's authorship preserved via cherry-pick.

Infographic

[github-actions]

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_{ran on 2b17db0}

ℹ️ Info

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1 visual diff.

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Failed to upload diff-665a0833239e-onboarding-overlay-diff.png with gh image (exit code 1): Error uploading /home/runner/work/_temp/e2e-evidence/diff-665a0833239e-onboarding-overlay-diff.png: step 0 (get upload token): uploadToken not found on repo page — do you have write access to NousResearch/hermes-agent? (or, if NousResearch enforces SAML SSO, authorize at https://github.com/orgs/NousResearch/sso)

[kandotrun]

Reproduced this regression on a live Slack gateway: the streamed text was delivered, then _handle_message_with_agent raised NameError: name '_history_media_paths' is not defined while dispatching a MEDIA: attachment, producing the generic unexpected-error follow-up.

I independently verified this patch with a RED/GREEN full-handler regression that included both an echoed prior-turn MEDIA: tag and a newly generated image:

• before the patch: the handler returned the generic unexpected-error response and delivered no attachment;
• after the patch: the handler completed normally, filtered the prior attachment, and delivered only the new image.

Local verification: 203 related gateway/media tests passed; blocking ruff, git diff --check, and bytecode compilation passed. Recomputing from the pre-turn history here preserves the intended cross-turn deduplication and fixes the out-of-scope reference.