flask: make it compatible with streaming responses in flask 3.1+

[fatelei]

Description

Add Flask 3.1+ compatibility with proper context cleanup for streaming responses to prevent memory leaks and token reuse

Fixes #3937

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• add unittest, reason: OpenTelemetry Context Detach Failure in Streaming Responses langgenius/dify#25673, fix flask: Context Detach Failure in Streaming Responses #3937

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[xrmx]

It looks to me that the diff is a lot bigger that what the fix would require, any chance that you can reduce that? We can do the refactoring later (or before) but all at the same commit is hard to review. Also we should add some tests with flask 3.1.

[fatelei]

It looks to me that the diff is a lot bigger that what the fix would require, any chance that you can reduce that? We can do the refactoring later (or before) but all at the same commit is hard to review. Also we should add some tests with flask 3.1.

change it, make small change