Compute total test coverage in CI

[efaulhaber]

This PR

• Shows two flags in the codecov PR comments:
  
• Shows the total flag in the README, replacing the embarrassing 64% badge by something like 90%.
• Makes the Ubuntu 1.11 run slower (because example tests are run with coverage). But it still only takes slightly longer than the Windows run, so I guess that's okay.
• Sets a reasonable unified tolerance for the dam break validation for all architectures and versions. Testing for zero error even failed in this PR for no apparent reason (just because coverage was enabled?). I think it makes more sense to test for a reasonable eps, which would be eps(1e5) = ~1.5e-11, since the pressure is in the order of 1e5.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.00%. Comparing base (f35ebe2) to head (04141e7).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #980       +/-   ##
===========================================
+ Coverage   64.45%   90.00%   +25.55%     
===========================================
  Files         117      118        +1     
  Lines        8122     8485      +363     
===========================================
+ Hits         5235     7637     +2402     
+ Misses       2887      848     -2039     

Flag	Coverage Δ
total	90.00% <ø> (?)
unit	64.58% <ø> (+0.12%)	⬆️

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[Copilot]

Pull Request Overview

This PR fixes coverage collection in CI and adds comprehensive test coverage tracking by computing both unit and total (unit + examples) coverage metrics. The key fix is correcting the Julia version check that was preventing any coverage from being collected.

• Fixes broken coverage collection by changing version check from '1' (which never matched) to '1.11'
• Adds total coverage tracking by enabling coverage for example tests and creating a separate "total" flag
• Simplifies validation tests by removing platform and version-specific tolerance conditions

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
.github/workflows/ci.yml	Fixes coverage collection condition and adds total coverage tracking with separate processing and upload steps for unit and total coverage flags
test/validation/validation.jl	Simplifies pressure validation tests by using uniform tolerance across all platforms and Julia versions

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