fix(rust): add heartbeat handle and drop for rust sdk

[slbotbm]

Which issue does this PR address?

Relates to #3733 (comment)

Rationale

The heartbeat handle was being discarded and on shutdown, not being stopped on calling shutdown.

What changed?

Adds a heartbeat_handler that is saved when connect() is called. In case connect is called repeatedly, the code uses the handler, and drops it when the client is destroyed.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

If AI tools were used, please answer:

1. Which tools? codex
2. Scope of usage? code generation
3. How did you verify the generated code works correctly? tested it
4. Can you explain every line of the code if asked? yes

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.10%. Comparing base (f900e7a) to head (48de5a8).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
core/sdk/src/clients/client.rs	75.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             master    #3852       +/-   ##
=============================================
- Coverage     76.76%   52.10%   -24.67%     
- Complexity     1021     1316      +295     
=============================================
  Files          1365     1376       +11     
  Lines        172679   152834    -19845     
  Branches     142608   121965    -20643     
=============================================
- Hits         132559    79630    -52929     
- Misses        36300    70013    +33713     
+ Partials       3820     3191      -629     

Components	Coverage Δ
Rust Core	45.19% <75.00%> (-30.61%)	⬇️
Java SDK	66.40% <ø> (+3.18%)	⬆️
C# SDK	61.44% <ø> (-14.69%)	⬇️
Python SDK	89.98% <ø> (ø)
PHP SDK	82.97% <ø> (ø)
Node SDK	96.36% <ø> (+0.08%)	⬆️
Go SDK	69.13% <ø> (ø)

Files with missing lines	Coverage Δ
core/sdk/src/clients/client.rs	88.74% <75.00%> (-0.48%)	⬇️

... and 449 files with indirect coverage changes

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