feat(go): add diagnostic event subscriptions to TCP client

[explise]

Which issue does this PR address

Closes #3219

Rationale

The TCP client had // TODO publish event stubs at the connect/disconnect/
shutdown paths with no way for callers to observe connection or auth state
changes. This fills those in and is groundwork for a high-level IggyConsumer
abstraction: diagnostic events let the consumer detect disconnects and re-join
consumer groups eagerly on reconnect. They are also useful on their own for
application-level observability (logging drops, health metrics).

What changed?

• DiagnosticEvent enum + String() in contracts/, and SubscribeEvents()
  on the Client interface.
• A small non-blocking broadcaster in the TCP client fanning out to multiple
  independent subscribers, with an idempotent unsubscribe function.
• Emission wired into connect (Connected), failed connect / invalidate /
  disconnect (Disconnected), shutdown (Shutdown, then channel close),
  login (SignedIn), logout (SignedOut) and leader redirection
  (Redirected).

Review feedback addressed

Rebased onto current master (adapting to the transportState/sessionState
split and the reworked register/settleOnLeader sign-in flow) and applied
the open review threads from #3621:

• Redirected is now published after the teardown, not before.
• Connected/Disconnected are published before releasing c.mtx.
• Broadcaster cleanups: plain send after draining a slot, no once.Do, no
  closed-guard in unsubscribe, slices.Delete, and the broadcaster is held
  by value.
• Integration tests use the SDK's existing env-based e2e helpers instead of a
  per-package testcontainers TestMain, removing the container-leak on
  startup failure.

Local Execution

• Passed. gofmt, go build ./..., go vet ./... clean.
• Unit tests pass (go test ./client/tcp/).
• Integration tests pass against a live apache/iggy:edge server: all 5
  TestDiagnosticEvents_* cases (signed-in, signed-out, shutdown + channel
  close, multiple subscribers, full lifecycle).
• Full module green with -race: IGGY_TCP_ADDRESS=... go test -race ./....

AI Usage

1. Tool: Claude Code.
2. Scope: Rebase/port onto current master, applying the prior review
   feedback, and test repair.
3. Verification: Local go build/go vet/gofmt, unit tests, and the
   integration suite run against a real server, including -race.
4. Can I explain every line? Yes.

[codecov]

Codecov Report

❌ Patch coverage is 72.22222% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.57%. Comparing base (df4c75d) to head (7054617).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
foreign/go/contracts/diagnostic_event.go	0.00%	16 Missing ⚠️
foreign/go/client/tcp/tcp_core.go	85.71%	2 Missing ⚠️
foreign/go/client/tcp/tcp_events.go	94.87%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #4120      +/-   ##
============================================
- Coverage     86.02%   85.57%   -0.45%     
  Complexity     1451     1451              
============================================
  Files          1248     1250       +2     
  Lines        194879   194949      +70     
  Branches     160177   160177              
============================================
- Hits         167637   166829     -808     
- Misses        23141    24062     +921     
+ Partials       4101     4058      -43     

Components	Coverage Δ
Rust Core	86.93% <ø> (ø)
Java SDK	67.56% <ø> (ø)
C# SDK	77.06% <ø> (ø)
Python SDK	91.33% <ø> (ø)
PHP SDK	85.65% <ø> (ø)
Node SDK	96.26% <ø> (ø)
Go SDK	56.27% <72.22%> (-13.13%)	⬇️

Files with missing lines	Coverage Δ
foreign/go/client/tcp/tcp_session_management.go	86.82% <100.00%> (+0.19%)	⬆️
foreign/go/client/tcp/tcp_core.go	87.39% <85.71%> (+0.64%)	⬆️
foreign/go/client/tcp/tcp_events.go	94.87% <94.87%> (ø)
foreign/go/contracts/diagnostic_event.go	0.00% <0.00%> (ø)

... and 39 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.