refactor: move attribute projection parsing to Swift

[nickolas-dimitrakas]

Background

MPAttributeProjection was the last unmigrated file in the projection/filter series, duplicating navigation logic MPProjectionFieldParser already implements.

What Has Changed

• Added attributeProjectionFields(from:attributeIndex:) to MPProjectionFieldParser.swift, reusing the existing private navigation so the two readers can't drift; coerces string-encoded data_type/is_required via MPJSONCoercion.
• Fixed a crash: a null entry in attribute_maps hit -[NSNull objectForKeyedSubscript:]; unreadable entries now fall back to the existing absent-key defaults.
• Added a bounds guard in configureProjections: for the unvalidated remote message_type index — preserves the existing (working) off-by-one that lets Media projections register, pinned by a new test.
• -setDataType:'s range clamp stays in ObjC (asserted by an existing test).

Validation

• abi-guard PASSED (no public ABI diff)
• trunk check clean
• ObjC 1004 / Swift 629 tests green on both schemes
• tvOS: CI only (no local simulator runtime)

Checklist

• Self-review completed
• Tests added or updated
• Tested locally

[github-actions]

🐦 Swift Migration Progress

Production implementation code at 013280fb5774 compared with bbb0af88e5ed.

Area	Goal	Progress	Base	This PR	Swift SLOC	Objective-C remaining	Change
Core SDK	Short term — in scope	█████▏░░░░	51.43%	51.43%	7,893	7,454	➖ 0.00 pp
Core SDK	Long term — all Objective-C	███▊░░░░░░	37.32%	37.32%	7,893	13,256	➖ 0.00 pp
SDK kit infrastructure	Short term — in scope	██████▊░░░	66.70%	66.96%	2,021	997	🚀 +0.26 pp
SDK kit infrastructure	Long term — all Objective-C	████░░░░░░	39.57%	39.95%	2,021	3,038	🚀 +0.38 pp
Standalone kits	Short term — in scope	▏░░░░░░░░░	1.06%	1.06%	152	14,218	➖ 0.00 pp
Standalone kits	Long term — all Objective-C	▏░░░░░░░░░	1.06%	1.06%	152	14,218	➖ 0.00 pp

Objective-C retained by design: Core SDK 5,802 · SDK kit infrastructure 2,041 · Standalone kits 0.

This PR's code movement

Area	Swift lines added	Objective-C lines removed
Core SDK	0	0
SDK kit infrastructure	55	10
Standalone kits	0	0

How this is measured

• Current composition uses production source lines of code (SLOC) from cloc; comments and blank lines are excluded.
• Short term — in scope excludes the Objective-C the migration will not delete, so 100% is the end of this project: every in-scope implementation gone.
• Long term — all Objective-C keeps the full denominator. Reaching 100% there means the public API itself becomes Swift, which is a breaking change reserved for a future major release.
• The gap between the two rows is the retained public/kit contract, runtime-identity, and boundary-glue surface listed in Tools/swift-migration-retained-objc.txt.
• Retained wrappers keep their Objective-C interface but still shed logic to Swift. That thinning moves the long-term row and the retained figure, not the short-term row.
• Both revisions are measured with the manifest from the head revision, so a manifest edit does not by itself move the reported change. A retained file this pull request renamed or deleted still counts as retained at the base.
• Pull request movement uses physical additions/deletions from git diff base...head --numstat; it counts retained files too and is intentionally separate from SLOC totals.
• Core excludes SDK kit infrastructure and vendored libraries. Standalone kits include only files below Kits/**/Sources.
• Tests, examples, headers, build outputs, vendored libraries, and the MParticle/Sources Swift overlay are excluded.
• Objective-C++ (.mm) is included in the Objective-C figures and removed counts.

Generated with cloc 2.10. This report is informational and does not gate migration direction.

[github-actions]

📦 SDK Size Impact Report

Measures how much the SDK adds to an app's size (with-SDK minus without-SDK).

Metric	Target Branch	This PR	Change
App Bundle Impact	2.46 MB	2.47 MB	+16 KB
Executable Impact	848 bytes	848 bytes	+N/A
XCFramework Size	6.80 MB	6.82 MB	+16 KB

➡️ SDK size impact change is minimal.

Raw measurements

Target branch (workstation/swift-migration):

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2600,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2516,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6968}

This PR:

{"baseline_app_size_kb":84,"baseline_executable_size_bytes":75464,"with_sdk_app_size_kb":2616,"with_sdk_executable_size_bytes":76312,"sdk_impact_kb":2532,"sdk_executable_impact_bytes":848,"xcframework_size_kb":6984}

[cursor]

PR Summary

Medium Risk
Touches kit projection registration and remote-config parsing paths that drive event routing; behavior is mostly defensive but preserves a deliberate Media message-type quirk.

Overview
Moves data_type / is_required parsing for attribute projections out of MPAttributeProjection into MPProjectionFieldParser.attributeProjectionFields, shared with the existing attribute-map navigation so the two code paths stay aligned. MPAttributeProjection now applies those fields via the existing setDataType: clamp.

Fixes crashes on malformed remote config: null or non-dictionary attribute_maps entries no longer trigger -[NSNull objectForKeyedSubscript:]; they fall back to string type and not required, matching the old absent-key behavior.

configureProjections: now skips projections whose message_type is above messageTypeSize (logs and continues) so bogus indices cannot raise NSRangeException, while still allowing index messageTypeSize so Media projections keep the historical off-by-one append behavior. New ObjC/Swift tests cover null maps, invalid attribute_maps, and in/out-of-range message types.

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