refactor(go): separate transport state and session state

[chengxilo]

Which issue does this PR address?

Relates to #3651

Rationale

Mixed state in a tech debt. It doesn't represent the client state correctly. Instead of map the tech debt from Rust SDK, I want to address it before #3650

What changed?

1. Separate transport state and session state.

2. Now LoginUser/LoginPersonalAccessToken/Logout will be the only functions that control the session state.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Claude Opus 4.8
2. implementation
3. I manually reviewed, it's an small refactor and doesn't change logic, doesn't bring too much side effect
4. Yes

BREAKING CHANGE:

1. the exported iggcon.State type and its State* constants are removed, replaced by TransportState and SessionState.
2. The zero value of iggcon.State now means TransportStateDisconnected (not connected yet) rather than StateShutdown (permanently dead).
3. StateAuthenticating is dropped with no replacement

[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 56.25000% with 21 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.62%. Comparing base (c0218f1) to head (7f83902).

Files with missing lines	Patch %	Lines
foreign/go/contracts/state.go	0.00%	12 Missing ⚠️
foreign/go/client/tcp/tcp_core.go	68.00%	8 Missing ⚠️
foreign/go/client/tcp/tcp_session_management.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3652      +/-   ##
============================================
- Coverage     73.99%   73.62%   -0.37%     
  Complexity      937      937              
============================================
  Files          1301     1286      -15     
  Lines        147389   140602    -6787     
  Branches     122949   116513    -6436     
============================================
- Hits         109057   103519    -5538     
+ Misses        34861    33845    -1016     
+ Partials       3471     3238     -233     

Components	Coverage Δ
Rust Core	73.87% <ø> (-0.42%)	⬇️
Java SDK	62.44% <ø> (ø)
C# SDK	72.23% <ø> (+0.03%)	⬆️
Python SDK	91.65% <ø> (-0.52%)	⬇️
PHP SDK	84.29% <ø> (ø)
Node SDK	91.13% <ø> (-0.23%)	⬇️
Go SDK	42.87% <56.25%> (+0.15%)	⬆️

Files with missing lines	Coverage Δ
foreign/go/client/tcp/tcp_session_management.go	64.51% <90.90%> (+7.37%)	⬆️
foreign/go/client/tcp/tcp_core.go	57.84% <68.00%> (+0.70%)	⬆️
foreign/go/contracts/state.go	0.00% <0.00%> (ø)

... and 120 files with indirect coverage changes

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[hubcio]

i checked this PR + #3651 proposal, overall it looks ok. main concern is the concurrent login race below. also "seperate" -> "separate" in the PR title.

[chengxilo]

/ready