[iOS] Move mainSid calculation to SFOAuthCredentials

[wmathurin]

Summary

• Adds a mainSid computed property to SFOAuthCredentials that returns parentSid for JWT token format and accessToken otherwise
• Simplifies SalesforceWebViewCookieManager.swift (in the iOS-Hybrid repo) to use creds.mainSid directly — that change is in a separate draft PR (SalesforceMobileSDK-iOS-Hybrid) blocked on this one merging first
• Adds 2 unit tests to SFOAuthCredentialsTests.m covering both branches of the logic

GUS

• W-24023805

Test plan

• testMainSid_withNonJwtFormat_returnsAccessToken — passes (new)
• testMainSid_withJwtFormat_returnsParentSid — passes (new)
• Full SalesforceSDKCore test suite — no regressions

[github-actions]

	1 Warning
⚠️	Static Analysis found an issue with one or more files you modified. Please fix the issue(s).

Clang Static Analysis Issues

File	Type	Category	Description	Line	Col
SFOAuthCredentials	Dereference of null pointer	Logic error	Access to instance variable '_credentialsChangeSet' results in a dereference of a null pointer (loaded from variable 'self')	199	27
SFOAuthCredentials	Nullability	Memory error	nil passed to a callee that requires a non-null 1st parameter	304	20

Generated by 🚫 Danger

[github-actions]

	Tests	Passed	Skipped	Failed ❌️
AuthFlowTester UI Test Results all	1 ran			1 ❌

Test	Result
AuthFlowTester UI Test Results all
AuthFlowTesterUITests.xctest
LegacyLoginTests.testCAOpaque_DefaultScopes_WebServerFlow()	❌ failure

[github-actions]

	Tests	Passed ✅	Skipped	Failed
SalesforceSDKCore iOS ^18 Test Results	1011 ran	1011 ✅

Test	Result
No test annotations available

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.46%. Comparing base (7fb2f46) to head (33d3b55).
⚠️ Report is 4 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #4153      +/-   ##
==========================================
- Coverage   70.37%   67.46%   -2.91%     
==========================================
  Files         254      254              
  Lines       22849    22859      +10     
==========================================
- Hits        16080    15422     -658     
- Misses       6769     7437     +668     

Components	Coverage Δ
Analytics	70.78% <ø> (ø)
Common	70.88% <ø> (ø)
Core	61.12% <93.75%> (-4.37%)	⬇️
SmartStore	73.45% <ø> (ø)
MobileSync	88.84% <ø> (ø)

Files with missing lines	Coverage Δ
...lesforceSDKCore/Classes/OAuth/SFOAuthCredentials.m	92.00% <100.00%> (-0.96%)	⬇️

... and 29 files with indirect coverage changes

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[sfdctaka]

LGTM!