fix(share_plus): preserve the subject when sharing a URI on iOS

[kaluli123123]

Description

Preserve the existing share title/legacy subject when sharing a URI on iOS. The URI branch previously passed nil to the activity controller while the text and file branches forwarded the value. The NSURL share item and existing completion, excluded-activity and popover behavior are unchanged.

Added native XCTest coverage through plugin registration and a binary MethodChannel message. The tests inspect a real UIActivityViewController for the supplied subject, title precedence, and omitted subject. On unchanged production code they report 1 pass and 2 failures; with the fix all 3 pass on an iPhone 17 Pro simulator, iOS 26.5, arm64. The package's Dart tests pass 14 with 1 existing skip, analysis reports no issues, Swift strict formatting and formatting of the modified Objective-C regions pass. Independent read-only review found no blocking issues.

The Xcode 27 verification command uses ARCHS=arm64, deployment target 15 and an isolated test bundle id; these command-line overrides address the unchanged example's baseline build limitations. The published iOS 13 minimum is unchanged. Mail compose UI, physical devices, iPad rendering, iOS 13 runtime and the full cross-platform CI matrix remain unverified. The tests prove controller configuration, not email delivery.

Codex assisted implementation, tests and independent review. The GitHub-verified signed delivery tree matches the locally tested tree; this draft remains pending contributor and platform-maintainer review.

Related Issues

• Fixes [Bug]: (share_plus) iOS drops the subject when sharing a uri #3994.

Checklist

• I read the Contributor Guide and followed the package-specific process.
• I titled the PR using Conventional Commits.
• I did not modify CHANGELOG.md or plugin version files.
• All existing and new tests are passing. (The complete cross-platform matrix is not claimed; actual local results are above.)
• The package analyzer reports no problems.

Breaking Change

• Yes.
• No. No public API changes.

[kaluli123123]

The repository-wide analyzer failure is also present on the unchanged base commit 53e5ecf38c8775af21c62c2932d07f88bad1a3b2e08064d336.

• This PR's analyzer job reports 19 LocalPlatform/FakePlatform deprecation diagnostics in android_intent_plus and one in its example.
• The base branch's analyzer job reports the same diagnostics in those unchanged packages.
• share_plus, share_plus_platform_interface, and share_plus_example report no analyzer issues in both runs. The share-specific build and integration jobs passed in this PR.

This PR remains a draft pending maintainer review; it does not change the unrelated analyzer diagnostics.