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

[atharvalade]

Which issue does this PR close?

Closes #2866

Rationale

The Python SDK is built with maturin/PyO3, so all business logic is compiled Rust. The previous two attempts (PR #2790 with pytest-cov, PR #2817 with manual llvm-cov) both failed: pytest-cov cannot measure Rust code, and the manual llvm-cov approach produced 0% coverage due to missing CARGO_TARGET_DIR centralization and LCOV source paths relative to the crate root instead of the repo root.

What changed?

Coverage reports were empty because raw llvm-profdata/llvm-cov export produced paths like src/client.rs while Codecov expected foreign/python/src/client.rs under the python flag. Dependency code from core/sdk also leaked into the report with no filtering.

The fix rewrites both post-merge and pre-merge coverage pipelines to use cargo-llvm-cov properly (matching the existing Rust coverage job pattern and the cjermain/rust-python-coverage reference). It sources env vars via cargo llvm-cov show-env, centralizes CARGO_TARGET_DIR, drops --release from maturin develop, generates LCOV with cargo llvm-cov report, filters dependencies with --ignore-filename-regex, and fixes paths with sed.

Local Execution

• Passed
• Pre-commit hooks ran

AI Usage

1. Opus 4.6
2. research, planning
3. Verified by reviewing against the cjermain/rust-python-coverage reference repo, cross-checking env var safety with the existing Rust coverage job, and confirming Codecov path matching with codecov.yml flag configuration
4. Yes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.40%. Comparing base (62f7256) to head (252633a).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2867      +/-   ##
============================================
+ Coverage     67.85%   68.40%   +0.54%     
  Complexity      739      739              
============================================
  Files          1052     1052              
  Lines         84635    84635              
  Branches      61213    61224      +11     
============================================
+ Hits          57432    57892     +460     
+ Misses        24833    24363     -470     
- Partials       2370     2380      +10     

Flag	Coverage Δ
csharp	67.43% <ø> (-0.19%)	⬇️
go	6.33% <ø> (ø)
java	54.83% <ø> (ø)
node	92.26% <ø> (-0.15%)	⬇️
python	81.57% <ø> (+81.57%)	⬆️
rust	70.08% <ø> (-0.03%)	⬇️

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see 24 files with indirect coverage changes

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[atharvalade]

pls check there's 2 flaky tests