Tracking: explicit connection selection + UPS transport headers

[HansDaigle]

Summary

• Add carrier_id and test_mode to TrackingData so POST tracking APIs can explicitly select the target connection (consistent with shipping connection resolution behavior).
• Wire those selectors into both proxy and manager tracking POST endpoints (carrier_filter).
• Update UPS tracking proxy to auto-populate required transport headers:
  • transId (generated UUID per request)
  • transactionSrc (karrio-test/karrio-prod from test_mode)

Why

• Tracking requests can target the wrong connection when multiple accounts exist for a carrier.
• UPS tracking can fail with TV0001/TV0011 when transport headers are missing.

Tests

• Added proxy tracking test to assert carrier_id/test_mode are forwarded to Shipments.track.
• Added manager trackers test to assert connection selectors reach Connections.first.
• Updated UPS tracking test to assert transactionSrc and UUID transId headers are sent.

Notes

• Validated syntax locally (py_compile) for changed files.
• Full server test suite was not run in this environment because Karrio server dependencies/runtime are not bootstrapped here.

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[HansDaigle]

Superseded by split PRs for easier review:\n- #976 (tracking connection selection)\n- #977 (UPS transId/transactionSrc headers)\n\nClosing this combined PR in favor of the two scoped changes.

[HansDaigle]

Closing in favor of #976 and #977.