fix(antigravity): strip billing header from system instruction before upstream call

[sususu98]

Summary

• Strip x-anthropic-billing-header block from the Claude system array before forwarding to Gemini upstream
• This block is client-internal metadata and should not be included in systemInstruction.parts

Test plan

• Verify requests with billing header in system instruction no longer forward it upstream
• Verify normal system instructions are unaffected

[gemini-code-assist]

Code Review

This pull request introduces a filter to skip system prompts starting with 'x-anthropic-billing-header:' during Claude request translation. The review feedback suggests improving the robustness of this check by normalizing the input string (trimming whitespace and lowercasing) and applying the same logic to other request formats for consistency.

[gemini-code-assist]

The check for the billing header is case-sensitive and does not account for potential leading or trailing whitespace. Normalizing the string before checking the prefix makes the stripping more robust. Additionally, this logic should also be applied to the case where the system instruction is a single string (lines 115-119) to ensure consistent behavior across different request formats.

Suggested change

	if strings.HasPrefix(systemPrompt, "x-anthropic-billing-header:") {
	continue
	}
	if strings.HasPrefix(strings.ToLower(strings.TrimSpace(systemPrompt)), "x-anthropic-billing-header:") {
	continue
	}