fix(harmony-renderer): cap system identity instructions at 4096 chars

[SURUJ404]

Long system instructions injected into the Harmony model identity could produce unexpectedly large prompts with no upper bound. Truncate at 4096 characters with a clear '... [truncated]' suffix.

This is a follow-up improvement to #46800 (Harmony Renderer for GPT-OSS).

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[SURUJ404]

This adds a 4096-character cap on system instructions injected into the Harmony model identity in system_content(). Without this cap, a user could pass arbitrarily long system instructions (e.g., via a system or developer message), which would balloon the prompt with no upper bound. The truncation appends "... [truncated]" so it's clear when truncation occurred.

Implementation follows the principle that the system identity should be bounded — it's metadata about the model, not arbitrary user content. The 4096 limit is generous enough for any reasonable use case but prevents pathological inputs.