fix: append streamed content deltas instead of replacing previous content

[Osamaali313]

Problem

agno streams RunContent as token-level deltas — RunContentEvent is documented as "Main event for each delta of the RunOutput", and providers emit per-chunk delta text (e.g. the Anthropic provider yields RunContentEvent(content=text_delta)).

The stream handler instead treated chunk.content as cumulative:

const uniqueContent = chunk.content.replace(lastContent, '')
lastMessage.content += uniqueContent
lastContent = chunk.content

String.replace(lastContent, '') removes the first occurrence of the previous delta from the current delta. Whenever a delta repeats text from the previous one — common with repeated words or tokens — the current delta is mangled.

Reproduction

Running the exact reducer over a delta stream (agno deltas, left→right):

streamed deltas	rendered (before)	expected
['na', ' na', ' na']	"na "	"na na na"
['go', ' go']	"go "	"go go"
['the', ' the']	"the "	"the the"
['Hello', ' world']	"Hello world"	"Hello world" (unaffected)

Fix

Append each delta directly and drop the unused lastContent tracking:

lastMessage.content += chunk.content

Non-repeating content is unchanged; repeated words/tokens are no longer dropped. The JSON-content branch already appended its block, so only its dead lastContent assignment is removed.

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