Fix grpc eof wrapping

[ljluestc]

What problem does this PR solve?

Issue Number: Close #9390

What is changed and how does it work?

server: fix io.EOF wrapping in gRPC streams (heartbeatServer, tsoServer, bucketHeartbeatServer) to ensure correct error comparison

The `bucketHeartbeatServer`, `tsoServer`, and `heartbeatServer` wrappers in `pd/server/grpc_service.go` were unconditionally wrapping errors returned by `Recv()` and `Send()` with `errors.WithStack`. This included `io.EOF`, which is a sentinel error used to signal the end of a stream.

When `io.EOF` is wrapped, standard equality checks (`err == io.EOF`) fail. Upstream callers (like `server.recv` in `ReportBuckets`) that expect to handle stream termination gracefully instead receive a generic error, potentially causing incorrect error handling flow or unnecessary noise in logs.

I modified `pd/server/grpc_service.go` to explicitly check for `io.EOF` in the error handling blocks of these methods. If `io.EOF` is detected, it is returned directly without wrapping.

Verification

I added 5 new regression test cases in server/grpc_service_test.go to verify correct io.EOF handling and state management for each affected server wrapper. These tests mock the stream to return io.EOF and assert that the wrapper returns it unwrapped and sets the closed state.

• TestBucketHeartbeatServerRecvEOF: Verify bucketHeartbeatServer Recv behavior.
• TestHeartbeatServerSendEOF: Verify heartbeatServer Send behavior.
• TestHeartbeatServerRecvEOF: Verify heartbeatServer Recv behavior.
• TestTsoServerSendEOF: Verify tsoServer Send behavior.
• TestTsoServerRecvEOF: Verify tsoServer recv behavior.

To run the verification tests:

go test -v ./server/ -run "Test.*EOF"

Check List

Tests

• Unit test

Code changes

• No code

Side effects

• No side effects

Related changes

• No related changes

Release note

server: fix io.EOF wrapping in gRPC streams (heartbeatServer, tsoServer, bucketHeartbeatServer) to ensure correct error comparison

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The list of commits missing DCO signoff:

• 4aba0c6 server: fix wrong usage of gRPC interface (io.EOF wrapping) (wrong usage of gRPC interface #9390)
• c02954f docs: improve PR description readability and formatting
• 621298c fix prd
• d6b1b48 docs: restore PR description
• a830339 fix prd
• 84d76dc test: add integration test for gRPC stream EOF and update PR description

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[codecov]

Codecov Report

❌ Patch coverage is 86.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.66%. Comparing base (1b40b8d) to head (84d76dc).
⚠️ Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #10190      +/-   ##
==========================================
- Coverage   78.73%   78.66%   -0.08%     
==========================================
  Files         522      521       -1     
  Lines       70090    70052      -38     
==========================================
- Hits        55188    55108      -80     
- Misses      10935    11000      +65     
+ Partials     3967     3944      -23     

Flag	Coverage Δ
unittests	78.66% <86.66%> (-0.08%)	⬇️

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