docs(connectors): align MongoDB sink duplicate-key semantics

[avirajkhare00]

Summary

Align the MongoDB sink README and one stale integration test name with the behavior that shipped in #2815.

What changed

• document the composite MongoDB _id format
• document duplicate-key handling as idempotent replay tolerance
• keep insert-only and non-upsert wording explicit
• rename the misleading duplicate-key integration test

Why

The final merged MongoDB sink behavior in #2815 uses composite _id values, unordered inserts, and duplicate-key replay tolerance, but the README and one test name still reflected the older hard-failure wording.

Verification

• cargo test -p integration --test mod --no-run
• Docker-backed MongoDB integration tests were not run in this environment

Related

• Discussion: how should sinks handle partial external writes and replay-safe progress? #2940 for the broader partial-write and replay-safe progress discussion

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.21%. Comparing base (6812907) to head (686d7bc).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #2941      +/-   ##
============================================
- Coverage     70.24%   70.21%   -0.04%     
+ Complexity      909      763     -146     
============================================
  Files          1033     1045      +12     
  Lines         85446    86850    +1404     
  Branches      63030    63129      +99     
============================================
+ Hits          60023    60980     +957     
- Misses        22928    23344     +416     
- Partials       2495     2526      +31     

Flag	Coverage Δ
rust	70.45% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.
see 78 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.