refactor: use strings.Builder for message merging in oaistream

[dgageot]

Consecutive same-role messages in the OpenAI stream handler were being merged by concatenating strings with += in a loop. This causes repeated allocations and copying as the accumulated content grows.

The MergeConsecutiveMessages function now uses strings.Builder instead, which buffers the concatenation and reduces allocations to a single final operation. The change is strictly behavior-preserving: the condition mergedContent.Len() > 0 replaces mergedContent != "", and mergedContent.String() yields the same accumulated value.

Testing confirmed the change passes go build ./..., unit tests in pkg/model/provider/oaistream/, and golangci-lint with zero issues.

[docker-agent]

Assessment: 🟢 APPROVE

The strings.Builder refactor in MergeConsecutiveMessages is behaviorally equivalent to the original += concatenation in all code paths:

• var mergedContent strings.Builder is declared inside the per-group if block, so it is freshly zero-valued on every outer-loop iteration — no cross-group state leakage.
• mergedContent.Len() > 0 is semantically identical to mergedContent != "" for the newline-separator guard.
• mergedContent.String() on a never-written builder returns "", matching the old zero-value string — so calls to openai.SystemMessage("") / openai.UserMessage("") when only multi-content parts exist are unaffected.
• WriteString("") (for an empty str returned by getStringContent) is a no-op, matching the old mergedContent += "" behaviour.

No bugs or regressions identified. The change correctly reduces allocations without altering observable behaviour.