fix(go): complete TCP client teardown when conn.Close fails

[chengxilo]

Which issue does this PR address?

Relates to #3652 (comment)
Relates to #3621 (comment)

Rationale

Golang SDK TCP client return the error directly when c.conn.Close() return an error. This stop it from finishing the disconnect and shutdown process.

What changed?

1. instead of return err directly, we keep the error, finish the process of shutdown/disconnect, return the error
2. when close the connection, set the c.conn to nil
3. In disconnect method, now we return nil directly when the status is shutdown too.(or caller can use disconnect->connect to revive a client that was shutdown before)

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Claude Opus 4.8
2. implementaiton
3. Reviewed it
4. yes

[github-actions]

Thanks for the PR. It is labeled S-waiting-on-review and queued for review.

Slash commands (own line, regular comment) move it around the queue:

• /ready - back to S-waiting-on-review after addressing feedback
• /author - flip to S-waiting-on-author while you finish changes
• /request-review @user-or-team - request a reviewer

See CONTRIBUTING.md for details.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.50%. Comparing base (02c5e4f) to head (5d4d5a3).

Files with missing lines	Patch %	Lines
foreign/go/client/tcp/tcp_core.go	66.66%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3670      +/-   ##
============================================
- Coverage     73.59%   73.50%   -0.09%     
  Complexity      937      937              
============================================
  Files          1297     1291       -6     
  Lines        143769   141555    -2214     
  Branches     119330   117114    -2216     
============================================
- Hits         105811   104056    -1755     
+ Misses        34591    34276     -315     
+ Partials       3367     3223     -144     

Components	Coverage Δ
Rust Core	73.71% <ø> (-0.14%)	⬇️
Java SDK	62.44% <ø> (ø)
C# SDK	72.21% <ø> (+0.01%)	⬆️
Python SDK	92.17% <ø> (ø)
PHP SDK	84.29% <ø> (ø)
Node SDK	91.35% <ø> (+0.12%)	⬆️
Go SDK	42.71% <66.66%> (+0.43%)	⬆️

Files with missing lines	Coverage Δ
foreign/go/client/tcp/tcp_core.go	57.14% <66.66%> (+6.09%)	⬆️

... and 23 files with indirect coverage changes

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