fix(client): feature flags header should not serialize private fields

[shijiesheng]

What changed?

• Feature flags header's serializer should ignore all private fields (so it's not serialized into the blob)
• Also bundled other test fixes to bring all tests to green again chore(test): fix test flakiness due to tchannel span assertion #1077
  ** disabled several TChannel tests with trace enabled. It won't work for OSS server due to the grpc constraint. We are sunsetting V3 anyway; internally we have a grpc-go fork that allows forwarding headers like $trace$-xxx
  ** flaky tests due to race conditions of TChannel span activation.

Why?

Java's thrift encoder would add private fields like "__isset_bitfield". These additional fields cause the server to ignore all feature flags due to a bug (fix in the server cadence-workflow/cadence#8421)

How did you test it?

Unit Test + Integration Test

Potential risks

Release notes

Documentation Changes

[gitar-bot]

Code Review ✅ Approved

Updates the feature flags serializer to ignore private fields and includes test stability fixes. No issues found.

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