fix: strip leading {} from streaming tool call arguments (Bedrock)

[anton-tw]

Summary

• Fixes an issue where some providers (e.g., Bedrock) return an initial empty {} followed by actual arguments when streaming tool calls, resulting in malformed {}{...} JSON
• Detects and strips the leading {} prefix while preserving legitimate empty {} arguments

Test plan

• Added test for stripping {} prefix when followed by real arguments
• Added test to verify legitimate empty {} arguments are preserved

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Latest commit: acc60c2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@openai/agents-openai	Patch
@openai/agents	Patch

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[anton-tw]

@seratch We ran into an issue when using Bedrock as the provider - during streaming, tool call arguments would arrive as {} in the first chunk, followed by the actual arguments in subsequent chunks. This resulted in malformed JSON like {}{\"key\":\"value\"} which breaks parsing.

This PR adds a small fix that detects and strips the leading {} when it's followed by real arguments, while still preserving legitimate empty {} arguments for tools that don't require parameters.

Would appreciate your thoughts on this approach!