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 45.83333% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.49%. Comparing base (5a5bc9a) to head (38d76d2).

Files with missing lines	Patch %	Lines
foreign/go/client/tcp/tcp_core.go	48.00%	13 Missing ⚠️
foreign/go/contracts/state.go	0.00%	12 Missing ⚠️
foreign/go/client/tcp/tcp_session_management.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #3652   +/-   ##
=========================================
  Coverage     73.49%   73.49%           
  Complexity      937      937           
=========================================
  Files          1291     1291           
  Lines        141568   141572    +4     
  Branches     117128   117128           
=========================================
+ Hits         104046   104054    +8     
  Misses        34300    34300           
+ Partials       3222     3218    -4     

Components	Coverage Δ
Rust Core	73.71% <ø> (ø)
Java SDK	62.44% <ø> (ø)
C# SDK	72.21% <ø> (ø)
Python SDK	92.17% <ø> (ø)
PHP SDK	84.29% <ø> (ø)
Node SDK	91.35% <ø> (ø)
Go SDK	42.41% <45.83%> (+0.13%)	⬆️

Files with missing lines	Coverage Δ
foreign/go/client/tcp/tcp_session_management.go	64.51% <90.90%> (+7.37%)	⬆️
foreign/go/contracts/state.go	0.00% <0.00%> (ø)
foreign/go/client/tcp/tcp_core.go	51.60% <48.00%> (+0.55%)	⬆️

... and 1 file 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.