Add more Encoding testing

[tarekgh]

The PR 111320 introduced a new encoding test to address a regression case involving multi-byte encoding when decoding Unicode surrogate pairs. The test specifically uses the GB18030 code page. It would be beneficial to extend this test to cover all multi-byte encodings that support surrogate pairs.

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-text-encoding
See info in area-owners.md if you want to be subscribed.

[ericstj]

Do we also need to cover more methods on encoding with similar scenarios, or is this test covering enough? My motivation behind asking for this was to ensure we keep the framework working correctly with non-default encodings and common inputs for those encodings.

[tarekgh]

Do we also need to cover more methods on encoding with similar scenarios, or is this test covering enough?

I think covering the encoders/decoders too when handling surrogate pairs will be good. Also, we may add more test data to the existing tests too.

[tarekgh]

Considering that GB18030 is the only codepage (other than Unicode encodings) that supports mapping to surrogate pairs, and we already have tests covering the other encoding APIs, we are closing this issue. However, we will continue to monitor for any gaps in the tests and add coverage as needed if any are identified.