fix: Type check SentrySpan

[noahsmartin]

In a number of places took a value of type protocol SentrySpan and tried to use it as the SentrySpan class or the SentryTransaction class. ObjC doesn't really care about this, only Swift would tell you it is unsafe. ObjC seems to be particularly bad at giving a warning for this when there are multiple levels of protocols (as there are with SentrySpan and SentrySerializable).

This PR converts SentrySpan protocol in V9 to not use SentrySerializable, and instead just define the method directly. This causes the ObjC compiler to give us a warning every time we did this unsafe operation, which I've added explicit casts for. It doesn't change any behavior, but it will help us catch this next time because CI will emit an error in the V9 check, helping us to avoid introducing more of these unsafe casts.

#skip-changelog

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (main@5db87fa). Learn more about missing BASE report.
⚠️ Report is 12 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #5982   +/-   ##
========================================
  Coverage        ?   86.716%           
========================================
  Files           ?       424           
  Lines           ?     36716           
  Branches        ?     17366           
========================================
  Hits            ?     31839           
  Misses          ?      4829           
  Partials        ?        48           

Files with missing lines	Coverage Δ
Sources/Sentry/SentryBuildAppStartSpans.m	100.000% <100.000%> (ø)
Sources/Sentry/SentryCoreDataTracker.m	97.959% <100.000%> (ø)
Sources/Sentry/SentryCrashIntegration.m	99.200% <100.000%> (ø)
Sources/Sentry/SentryTimeToDisplayTracker.m	100.000% <100.000%> (ø)
Sources/Sentry/SentryTracer.m	97.213% <100.000%> (ø)
.../Sentry/SentryUIViewControllerPerformanceTracker.m	99.450% <100.000%> (ø)

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1222.98 ms	1248.47 ms	25.49 ms
Size	23.75 KiB	927.57 KiB	903.82 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
cc7f629	1226.00 ms	1245.51 ms	19.51 ms
80a5166	1224.49 ms	1251.29 ms	26.80 ms
82f60cf	1218.65 ms	1238.52 ms	19.87 ms
b9aacb6	1230.42 ms	1251.00 ms	20.58 ms
2691350	1224.92 ms	1255.82 ms	30.90 ms
9080e6c	1221.17 ms	1247.87 ms	26.71 ms
326984b	1235.06 ms	1252.75 ms	17.69 ms
2a36c3f	1227.27 ms	1246.89 ms	19.63 ms
916edbe	1243.86 ms	1256.11 ms	12.25 ms
324c109	1228.35 ms	1252.47 ms	24.12 ms

App size

Revision	Plain	With Sentry	Diff
cc7f629	23.75 KiB	878.48 KiB	854.73 KiB
80a5166	23.75 KiB	904.53 KiB	880.78 KiB
82f60cf	23.75 KiB	913.63 KiB	889.88 KiB
b9aacb6	23.75 KiB	913.64 KiB	889.89 KiB
2691350	23.75 KiB	850.73 KiB	826.98 KiB
9080e6c	23.75 KiB	926.80 KiB	903.05 KiB
326984b	23.74 KiB	926.64 KiB	902.90 KiB
2a36c3f	23.75 KiB	874.45 KiB	850.71 KiB
916edbe	23.75 KiB	908.41 KiB	884.66 KiB
324c109	23.75 KiB	919.91 KiB	896.16 KiB

Previous results on branch: fixSentrySpanTypeChecks

Startup times

Revision	Plain	With Sentry	Diff
a74512d	1219.81 ms	1252.22 ms	32.42 ms
1e63689	1225.76 ms	1243.73 ms	17.97 ms
cf7575c	1229.65 ms	1253.28 ms	23.64 ms

App size

Revision	Plain	With Sentry	Diff
a74512d	23.75 KiB	926.63 KiB	902.88 KiB
1e63689	23.75 KiB	926.63 KiB	902.88 KiB
cf7575c	23.75 KiB	926.63 KiB	902.88 KiB

[philprime]

LGTM