fix(ios): keep the podspec's build attributes out of the OpenCV branch

[msluszniak]

Description

Two defects in #1450, both reproduced against main at a1ff6c9.

if enable_opencv was never closed. Its end sat at the bottom of the spec, one line above install_modules_dependencies(s), and the indentation of everything after the s.dependency call reads as if the block had already ended. So five attributes were assigned only when OpenCV was on:

attribute	what an app loses without it
pod_target_xcconfig	every HEADER_SEARCH_PATHS entry
vendored_frameworks	ExecutorchLib.xcframework
static_framework	the use_frameworks! fix from #1448
libraries	z, sqlite3
frameworks	Accelerate, CoreML, Metal

An app that turns OpenCV off through the libs config block installs fine and then fails to compile or link. #1450 is what made this reachable: before it, a config block in an app package was ignored and OpenCV was always on.

rne_build_config was nil without a config file. The else branch set the five enable_* flags but never assigned the hash, and #1450 added rne_build_config["opencvPod"] on the path that branch reaches, so a checkout without rne-build-config.json raised NoMethodError during pod install. The comment above it names that case as supported ("a fresh checkout where the native libs were provisioned manually"). The branch is gone: the config is a hash either way, and every flag already reads != false, so an empty hash means all-enabled.

Test. pod install is the only thing that runs this file, and every example app takes the same config row, so a misplaced end passes lint, types, tests and review. __tests__/api/podspecAttributes.test.ts evaluates the real podspec under a stubbed Pod::Spec and asserts the build-critical attributes are assigned, for eight config rows. Against a1ff6c9 it fails three of them:

● with no config file (manual provisioning)
  podspec:242: undefined method `[]' for nil:NilClass (NoMethodError)
● with opencv off
● with every optional library off
  missing: pod_target_xcconfig, static_framework, vendored_frameworks, libraries, frameworks

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

yarn workspace react-native-executorch test __tests__/api/podspecAttributes.test.ts
git stash && yarn workspace react-native-executorch test __tests__/api/podspecAttributes.test.ts  # 3 failures

Needs an enableOpencv: false pod install on a device build before merge; I have only run the podspec evaluation.

Related issues

Follow-up to #1450.

Additional notes

Backport to release/0.10 goes on top of #1451.