feat(server-ng): add cluster.heartbeat_timeout for VSR liveness window

[hubcio]

The backup-side primary-liveness window was hardcoded at 5s
(NORMAL_HEARTBEAT_TICKS). On oversubscribed hosts scheduling stalls
exceed it, so backups elect a new primary although the old one is
healthy, and client sessions die with the view change - loaded
parallel integration runs failed 23 message_retrieval tests this
way.

Surface the window as [cluster] heartbeat_timeout on the server-ng
config. ClusterConfig is forked into server_ng_config (like the
tcp/quic/websocket forks) because the legacy struct sources its
defaults from the legacy config.toml, which is frozen for ng-only
knobs. The runtime converts the duration to consensus ticks and
installs it before init, a validator enforces a 2s floor (the zero
sentinels are rejected too), and a unit test pins the TOML default
to the consensus built-in. The retrieval scenarios now run with a
60s window so box load cannot fake primary death.

[codecov]

Codecov Report

❌ Patch coverage is 94.93333% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.40%. Comparing base (2fdd6e8) to head (32727fa).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/configs/src/server_ng_config/cluster.rs	96.72%	9 Missing and 1 partial ⚠️
core/server-ng/src/bootstrap.rs	59.09%	9 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3738       +/-   ##
=============================================
- Coverage     74.63%   60.40%   -14.23%     
  Complexity      950      950               
=============================================
  Files          1303     1303               
  Lines        149183   137150    -12033     
  Branches     124694   112735    -11959     
=============================================
- Hits         111338    82845    -28493     
- Misses        34348    50720    +16372     
- Partials       3497     3585       +88     

Components	Coverage Δ
Rust Core	57.51% <94.93%> (-17.41%)	⬇️
Java SDK	62.64% <ø> (ø)
C# SDK	71.13% <ø> (-1.16%)	⬇️
Python SDK	92.27% <ø> (ø)
PHP SDK	84.52% <ø> (ø)
Node SDK	92.70% <ø> (ø)
Go SDK	43.08% <ø> (ø)

Files with missing lines	Coverage Δ
core/configs/src/server_ng_config/defaults.rs	100.00% <100.00%> (ø)
core/configs/src/server_ng_config/server_ng.rs	47.05% <ø> (ø)
core/consensus/src/impls.rs	75.11% <100.00%> (+0.04%)	⬆️
core/consensus/src/lib.rs	0.00% <ø> (ø)
core/consensus/src/vsr_timeout.rs	94.67% <100.00%> (+0.09%)	⬆️
core/server-ng/src/cluster_meta.rs	15.87% <ø> (ø)
core/server-ng/src/http.rs	51.89% <ø> (ø)
core/server-ng/src/http/error.rs	31.86% <ø> (ø)
core/server-ng/src/partition_helpers.rs	64.74% <100.00%> (+0.10%)	⬆️
core/server-ng/src/bootstrap.rs	11.71% <59.09%> (+0.56%)	⬆️
... and 1 more

... and 259 files with indirect coverage changes

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