Introduce conversion between chrono::NaiveDate and DateTime avoiding RustOpaque

[TrackerSB]

Changes

Fixes #1833

Procedure and Checklist

In order to quickly iterate and avoid slowing down development pace by making CI pass, only the following simplified steps are needed, and I (fzyzcjy) will handle the rest of CI / moving the tests currently (will have more automation in the future).

• Implement the feature / fix the bug.
• Add tests in frb_example/dart_minimal.
• Make dart_minimal's CI green.

Utility commands

• Run codegen: cargo run --manifest-path /path/to/flutter_rust_bridge/frb_codegen/Cargo.toml -- generate
• Run tests: ./frb_internal test-dart-native --package frb_example/dart_minimal

[TrackerSB]

As far as I can see meanwhile (since my last attempt) the PR template changed in the way that I should have made an example in dart_minimal.
Unknowingly, I added the implementation and the tests in dart_pure and generated all the other variants. At least I guess I did that.

[codecov]

Codecov Report

❌ Patch coverage is 37.03704% with 34 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.40%. Comparing base (67bfaec) to head (90c9927).

Files with missing lines	Patch %	Lines
...library/codegen/generator/codec/sse/ty/delegate.rs	0.00%	8 Missing ⚠️
...st/spec_generator/codec/cst/decoder/ty/delegate.rs	0.00%	7 Missing ⚠️
...rt/spec_generator/codec/dco/decoder/ty/delegate.rs	0.00%	3 Missing ⚠️
..._codegen/src/library/codegen/ir/mir/ty/delegate.rs	0.00%	2 Missing ⚠️
...n/generator/api_dart/spec_generator/class/field.rs	0.00%	1 Missing ⚠️
.../codegen/generator/api_dart/spec_generator/info.rs	0.00%	1 Missing ⚠️
...c/library/codegen/generator/codec/sse/ty/record.rs	0.00%	1 Missing ⚠️
...rt/spec_generator/codec/cst/encoder/ty/delegate.rs	0.00%	1 Missing ⚠️
...dart/spec_generator/codec/cst/encoder/ty/record.rs	0.00%	1 Missing ⚠️
...t/spec_generator/codec/cst/encoder/ty/structure.rs	0.00%	1 Missing ⚠️
... and 8 more

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #2968       +/-   ##
===========================================
- Coverage   98.57%   68.40%   -30.18%     
===========================================
  Files         464      463        -1     
  Lines       19202    18589      -613     
===========================================
- Hits        18928    12715     -6213     
- Misses        274     5874     +5600     

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[fzyzcjy]

As far as I can see meanwhile (since my last attempt) the PR template changed in the way that I should have made an example in dart_minimal.
Unknowingly, I added the implementation and the tests in dart_pure and generated all the other variants. At least I guess I did that.

It is better to directly add things in pure_dart etc! :)

[TrackerSB]

I recognized that the regeneration of files resulted in automatically added explicit lifetime annotations. Is this expected, is it a problem, does it depend on the used Rust version?
Additionally, for some files the year in generated comments changed:

-            VALUE "LegalCopyright", "Copyright (C) 2025 com.example. All rights reserved." "\0"
+            VALUE "LegalCopyright", "Copyright (C) 2026 com.example. All rights reserved." "\0"

[TrackerSB]

What next steps are left for me to do to make this PR worth to be merged?

[fzyzcjy]

I recognized that the regeneration of files resulted in automatically added explicit lifetime annotations. Is this expected, is it a problem, does it depend on the used Rust version?
Additionally, for some files the year in generated comments changed:

Feel free to make a separate PR to firstly update code for these!

What next steps are left for me to do to make this PR worth to be merged?

I am sorry that I am having a very tight deadline recently :( I will try my best to find some time after the ddl to review and merge all pending PRs. At the same time, feel free to use your branch if you need this feature, as long as the critical tests in CI passes then usually the branch is good to use.