fix(ci): use cargo-llvm-cov for Python SDK coverage

[atharvalade]

Which issue does this PR close?

Closes #2816

Rationale

The Python SDK is built with maturin/PyO3, meaning all business logic is compiled Rust. pytest-cov can only measure Python source, so it produced empty reports and the Codecov upload silently failed.

What changed?

pytest-cov was generating empty coverage reports because there is no Python source code to instrument in a native Rust extension.

Replaced pytest-cov with cargo-llvm-cov, which instruments the Rust code at build time and collects profiling data when Python tests call into the compiled .so. This follows the same pattern used by pydantic-core and PyO3 itself.

Local Execution

• Not passed (CI-only workflow changes)
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. Researched how pydantic-core and PyO3 handle coverage, then scaffolded the workflow changes
3. Compared against pydantic-core and PyO3 CI workflows to verify the approach is correct
4. Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.57%. Comparing base (d5d31e5) to head (103d806).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2817      +/-   ##
============================================
- Coverage     68.07%   67.57%   -0.50%     
  Complexity      708      708              
============================================
  Files          1018     1027       +9     
  Lines         82550    83147     +597     
  Branches      60005    60015      +10     
============================================
- Hits          56194    56188       -6     
- Misses        24016    24608     +592     
- Partials       2340     2351      +11     

Flag	Coverage Δ
csharp	67.06% <ø> (-0.19%)	⬇️
go	6.84% <ø> (ø)
java	54.02% <ø> (ø)
node	92.26% <ø> (-0.04%)	⬇️
python	0.00% <ø> (?)
rust	69.61% <ø> (-0.02%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.
see 23 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.