[release/3.1] Handle shortened JSON file in dotnet openapi

[dougbu]

• manual backport of e23fd04 (without VS fixes)
  • also, no need to remove non-existent Resolve issues with dotnet-openapi tests in latest SDK #32686 test skips
• an existing JSON file must be truncated
• extend dotnet openapi refresh tests
  • include regression test for dotnet-openapi refresh does not truncate json documents. #35767

Addresses

#35767

Description

From #35767:

When dotnet-openapi is used to refresh a swagger.json document, if the new document is shorter than the old document, the file is not truncated to the new size leaving invalid trailing bytes that render it un-parsable, causing build errors.

Customer Impact

dotnet openapi refresh is very difficult to use (or justify using) in this scenario.

While the download numbers for Microsoft.dotnet-openapi aren't incredibly high (max. ~6500 for 5.0.6), this problem likely wasn't reported earlier only because OpenAPI documents tend to grow, not shrink. But, when customers hit this issue, they need to manually edit the extra-long file and rebuild. Not a positive experience.

Regression?

• Yes
• No

This has been a problem since the tool was first introduced in the run-up to v3.0.0.

Risk

• High
• Medium
• Low

Just correct the FileMode and we're done.

Verification

• Manual (required)
• Automated (new tests added)

Packaging changes reviewed?

• Yes
• No
• N/A

[ghost]

"Hi dougbu. Please make sure you've updated the PR description to use the Shiproom Template. Also, make sure this PR is not marked as a draft and is ready-to-merge.

To learn more about how to prepare a servicing PR click here.

[dougbu]

/ping reviewers