ci: Add WASM to Flutter test matrix

[Copilot]

Pull Request

• Report security issues confidentially.
• Any contribution is under this license.
• Link this pull request to an issue.

Issue

Closes: #1096

Approach

Adds WASM testing to the Flutter CI matrix as discussed in #1096.

• New matrix entry Flutter 3.38, Ubuntu, WASM with wasm: true flag
• Separate test step running flutter test --platform chrome --wasm
• Code coverage excluded for WASM runs (WASM tests don't produce standard coverage output)

Tasks

• Add tests
• Add changes to documentation (guides, repository pages, code comments)

Original prompt

Add wasm to the test matrix. See the last comments in the discussion in #1096.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 43.96%. Comparing base (828355d) to head (830a778).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1097   +/-   ##
=======================================
  Coverage   43.96%   43.96%           
=======================================
  Files          61       61           
  Lines        3596     3596           
=======================================
  Hits         1581     1581           
  Misses       2015     2015           

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