fix: Fix incorrect variable assignment for 'sampled' key

[xjshi]

In SentryTraceContext, the 'sampled' key was incorrectly using _sampleRate.

📜 Description

In SentryTraceContext.m, the 'sampled' key was incorrectly using _sampleRate. This resulted in malformed trace contexts being sent to Sentry, potentially breaking sampling decisions in distributed tracing.

[itaybre]

@xjshi thanks for the fix!

Would you mind adding a test for this? Or would like me to push the required tests for this? (to this doesn't happen again)

[codecov]

❌ 2 Tests Failed:

Tests completed	Failed	Passed	Skipped
4205	2	4203	13

View the top 1 failed test(s) by shortest run time

SentryTests.SentryFileManagerTests::testCreateDirectoryIfNotExists_successful_shouldNotLogError

Stack Traces | 0s run time

.../SentryTests/Helper/SentryFileManagerTests.swift:1056 - XCTAssertEqual failed: ("1") is not equal to ("0")

View the full list of 1 ❄️ flaky test(s)

SentryCrashReportStore_Tests::testDeleteAllReports

Flake rate in main: 99.12% (Passed 1 times, Failed 112 times)

Stack Traces | 0s run time

.../SentryTests/SentryCrash/SentryCrashReportStore_Tests.m:126 - ((sentrycrashcrs_getReportCount()) equal to (reportCount)) failed: ("3") is not equal to ("4")

To view more test analytics, go to the Test Analytics Dashboard
_{📋 Got 3 mins? Take this short survey to help us improve Test Analytics.}

[xjshi]

@itaybre Test added. It verifies all fields in the dictionary returned by the serialize method to ensure correct mapping.

[philprime]

@sentry review

[itaybre]

Updating the branch, so checks can pass 🟢

[itaybre]

LGTM

[itaybre]

Merging this PR, I will open a different one with the changelog changes