Perplexity Web (Pro/Max) only converted <tool>{...}</tool> text into
OpenAI tool_calls for non-streaming requests (hasTools && !stream).
Streaming requests -- the default for agentic coding clients -- got
the raw <tool> text as plain delta.content and never emitted a
tool_calls SSE delta, so clients could not execute tools.
Reuses the provider-agnostic buildToolModeResponse()/
toolCompletionToSseStream() helpers already shipped for chatgpt-web
(#5240): when tools are requested, buffer the full completion and
convert it into either a JSON completion or a terminal SSE replay
carrying delta.tool_calls + finish_reason: tool_calls, regardless of
the caller's stream flag. Extended buildToolModeResponse()'s idSeed
to be caller-supplied (default 'cgpt', perplexity-web passes 'pplx')
so tool_call ids stay provider-specific without duplicating the
helper. Non-tool streaming is unchanged (still lives token-by-token
via buildStreamingResponse).
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