"ultrathink" uses different tag name for thinking length limit

Bug Description

Different thinking mode triggers use different XML tag names for the same semantic concept:

• "think" / "think harder": uses <max_thinking_length>
• "ultrathink": uses <thinking_length_limit>

Both tags serve identical purposes (limiting tokens per thinking block) but have different names.

Observed Behavior

<!-- "think" trigger -->
<max_thinking_length>1024</max_thinking_length>

<!-- "ultrathink" trigger -->
<thinking_length_limit>60000</thinking_length_limit>

Expected Behavior

Consistent tag naming across all thinking modes. Single canonical tag name for this configuration.

Impact

• Suggests fragmented implementation
• Makes behavior harder to reason about
• Could cause issues if code paths handle tags differently