chore(bindings/dart): upgrade flutter_rust_bridge to 2.12.0 and fix native library loading

[TennyZhuang]

Which issue does this PR close?

No dedicated issue. This is a prerequisite infrastructure fix found during the OpenDAL binding regression effort — the existing Dart binding had a codegen/runtime version mismatch that caused dart test to fail entirely.

Rationale for this change

flutter_rust_bridge codegen was at 2.12.0 but the runtime dependency in pubspec.yaml and rust/Cargo.toml was pinned to 2.8.0. FRB enforces a codegen/runtime version match at startup, so the binding was broken for any consumer running dart test from a clean checkout.

This PR upgrades both sides to 2.12.0, regenerates all FRB-managed files, and fixes a macOS native library loading path that failed on arm64 native builds (cross-compilation target directory absent).

What changes are included in this PR?

• pubspec.yaml: flutter_rust_bridge: 2.8.0 → 2.12.0
• rust/Cargo.toml: flutter_rust_bridge = "=2.8.0" → "=2.12.0"
• Regenerated FRB bindings: frb_generated.dart, frb_generated.io.dart, frb_generated.web.dart, frb_generated.rs, api/opendal_api.dart
• lib/opendal.dart:
  • import 'dart:io' → import 'dart:io' as io to avoid name collisions after FRB 2.12 changes.
  • macOS native library loading fallback: check whether the cross-compilation target directory contains the expected library file, fall back to rust/target/release/ when not.
  • Internal Operator(scheme: schemeStr, ...) call updated to match FRB 2.12 generated parameter name. The public Storage.init({required String schemeStr, ...}) API is unchanged.

Are there any user-facing changes?

No. Storage.init(schemeStr: ...) public API is preserved. The internal generated Operator constructor parameter was renamed by FRB 2.12 codegen; the wrapper absorbs that change transparently.

Intentional divergence from Rust

None. This is a pure infrastructure fix.

Validation

Internal agent-to-agent pre-review used the required patch artifact flow before PR publication:

• Patch v1 artifact: id:499b4d7b-a96b-4e10-b321-bc9b980d5f33, SHA-256 f643e7baefc03a9d09c8471eaf0b66edff77727b461245f5f96f71590c246b29 — REQUEST CHANGES (Codex caught a source-breaking Storage.init(schemeStr:) → scheme: rename masquerading as infra-only; Commander caught anyhow dep being added in PR A but only used by PR B's read/write methods).
• Patch v2 artifact: id:13ca0fa7-99a9-4f16-a55e-2b5f2e1cb914, SHA-256 26c9a16c06eef524acc2976936ae4bf1b6533ab98aced99e31c43f0fc040e3bb — Commander APPROVED, Codex APPROVED.

Codex pre-review v2 verification (apache/opendal:agent-dart-frb-upgrade applied cleanly on origin/main):

• git diff --check / git show --check passed.
• Public Dart wrapper API preserved: Storage.init({ required String schemeStr, ... }) still exists and internally calls generated Operator(scheme: schemeStr, map: map).
• Tests still call Storage.init(schemeStr: ...); no public rename leak.
• bindings/dart/rust/Cargo.toml only bumps flutter_rust_bridge to =2.12.0; no misplaced anyhow dependency.

Reported handoff transcripts:

$ cd bindings/dart/rust && cargo build -r
   Finished release profile [optimized] target(s) in 2.18s

$ cd bindings/dart && dart test tests/opendal_test.dart
00:00 +0: loading tests/opendal_test.dart
00:00 +0: opendal fs schema File and Directory functions in fs schema
00:00 +1: opendal fs schema File and Directory functions in fs schema
00:00 +1: opendal memory schema File and Directory functions in memory schema
00:00 +2: opendal memory schema File and Directory functions in memory schema
00:00 +2: All tests passed!

Codex independent re-run reported the same cargo build -r success and dart test 2/2 pass.

Note: dart analyze is not currently clean in this binding due to a pre-existing reference in lib/src/rust/api/capability.dart to a generated API that is not present. That is unrelated to this PR and is not cited as validation.

AI Usage Statement

This patch was produced by AI agents during an OpenDAL binding regression effort. It was reviewed by separate AI reviewer/commander agents through Slock before publication:

• Produced by @od-kimi-scout-050121 (Kimi) as Dart Phase 1 infra worker.
• Pre-reviewed by @od-claude-lead-050121 (Claude, commander) — APPROVED v2.
• Pre-reviewed by @od-codex-review-050121 (Codex) — APPROVED v2 with verified SHA-256 26c9a16c06eef524acc2976936ae4bf1b6533ab98aced99e31c43f0fc040e3bb.
• Pushed under shared TennyZhuang auth per the regression-effort hybrid publishing policy.

PR B (Dart File.read/write Phase 1 feature, depends on this PR) follows separately, mapped to #7467.