fix(rust,sdk): stop late offset stores from hitting a left group

[spetz]

IntervalOrEach/Each/Every auto-commit modes queue offset stores
through a background channel task instead of storing them inline.
shutdown() left the consumer group right after a direct final
flush, with no idea the channel task still had a backlog. Under
real throughput the backlog landed after the group was already
left, and the broker rejected every one of them as an unknown
group member (5006, consumer_group_member_not_found).

shutdown() now closes the channel and wakes the periodic commit
loop first, awaiting both tasks before leaving the group. The
wait is bounded by a new configurable offset_drain_timeout (5s
default, set via IggyConsumerBuilder). On timeout the task is
aborted outright instead of left detached, so a slow drain can
no longer fire a stale store after the leave call returns.

Also cleared joined_consumer_group unconditionally after the
leave attempt. A failed leave RPC used to leave it stuck true.

[codecov]

Codecov Report

❌ Patch coverage is 47.05882% with 27 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.25%. Comparing base (5a5bc9a) to head (6a46cd8).

Files with missing lines	Patch %	Lines
core/sdk/src/clients/consumer.rs	51.16%	21 Missing ⚠️
core/sdk/src/clients/consumer_builder.rs	25.00%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #3668      +/-   ##
============================================
- Coverage     73.49%   73.25%   -0.25%     
  Complexity      937      937              
============================================
  Files          1291     1291              
  Lines        141568   141312     -256     
  Branches     117128   116944     -184     
============================================
- Hits         104046   103512     -534     
- Misses        34300    34498     +198     
- Partials       3222     3302      +80     

Components	Coverage Δ
Rust Core	73.48% <47.05%> (-0.23%)	⬇️
Java SDK	62.44% <ø> (ø)
C# SDK	71.04% <ø> (-1.17%)	⬇️
Python SDK	92.17% <ø> (ø)
PHP SDK	84.29% <ø> (ø)
Node SDK	91.26% <ø> (-0.10%)	⬇️
Go SDK	42.28% <ø> (ø)

Files with missing lines	Coverage Δ
core/sdk/src/clients/producer.rs	64.08% <ø> (-1.39%)	⬇️
core/sdk/src/clients/producer_dispatcher.rs	93.95% <ø> (ø)
core/sdk/src/clients/consumer_builder.rs	60.24% <25.00%> (-1.85%)	⬇️
core/sdk/src/clients/consumer.rs	65.51% <51.16%> (-1.07%)	⬇️

... and 50 files with indirect coverage changes

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