fix: ugprade openai package from v5 to v6

[seratch]

This pull request upgrades underlying openai package from 5.x to 6.x for 0.2.x series. This should have been done but I missed the change and unfortunately the integration tests didn't detect this necessary changes.

After releasing 0.2.0, I found:

• the zod 4 support is still incomplete
  • runtime: a bug with agent output type exists; this PR resolves it
  • tsc: MCP SDK still sorely relies on zod 3.x, so supporting only v4 is not yet an option
  • tsc: This SDK's public interface relies on zod 3.x object interface; the latter one needs to be changed to embrace both zod 4 and 3 interfaces

This is a necessary change, so we'll merge this. The remaining tasks are:

• continue research for zod 4 full support
• check MCP SDK updates for zod support
• reivist the integration tests to better detect this type of issues

[changeset-bot]

🦋 Changeset detected

Latest commit: fbfe587

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

This PR includes changesets to release 5 packages

Name	Type
@openai/agents-openai	Patch
@openai/agents-core	Patch
@openai/agents	Patch
@openai/agents-extensions	Patch
@openai/agents-realtime	Patch

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