Replace hardcoded timeouts with AI gateway timeout setting value

What does this MR do and why?

Replaces hardcoded timeouts with the AI gateway timeout setting (default: 60 seconds, minimum: 60 seconds, maximum: 600 seconds) that can be controlled at the admin/instance level.

References

Replace hardcoded timeouts with setting value (#579184)

Bug issue: Make 60s timeout applied by AIGW when making re... (#567878)

How to set up and validate locally

• Update the ai_gateway_timeout_seconds setting in the Rails console (MR to add the setting to the UI is still in progress: Add AI gateway timeout setting to admin page (!211496)):

setting = Ai::Setting.instance
setting.update!(ai_gateway_timeout_seconds: 100)

# Verify the change
setting.ai_gateway_timeout_seconds

• I tested this with Web Agentic Chat and Code Suggestions on the Web IDE and did not see any issues.

MR acceptance checklist

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