fix tissues to start at atmosphere partials instead of 0

[dylan-smith]

This pull request includes a small but significant change to the Tissue class in the dive-planner-service. The change ensures that the default values for PN2 and PHe are set to the environmental constants instead of zero.

[github-actions]

Unit Test Results

6 tests  ±0   6 ✅ ±0   0s ⏱️ ±0s
1 suites ±0   0 💤 ±0 
1 files   ±0   0 ❌ ±0 

Results for commit 6e182cb. ± Comparison against base commit ab809e1.

♻️ This comment has been updated with latest results.

[github-actions]

UI Test Results

1 files  ±0  9 suites  ±0   2m 56s ⏱️ -10s
3 tests ±0  3 ✅ ±0  0 💤 ±0  0 ❌ ±0 
9 runs  ±0  9 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 6e182cb. ± Comparison against base commit ab809e1.

♻️ This comment has been updated with latest results.

[github-actions]

This PR has been deployed to a preview site for testing.

This site will automatically be deleted when the PR is merged.

Last deployed: 2025-05-21T21:50:47Z (UTC)

[github-actions]

Unit Tests Code Coverage

Package	Line Rate	Branch Rate	Health
src	100%	100%	✔
src.app	100%	100%	✔
src.app.application-insights-service	12%	100%	❌
src.app.ceiling-chart	0%	0%	❌
src.app.change-depth	0%	0%	❌
src.app.change-gas	0%	0%	❌
src.app.current-stats	0%	0%	❌
src.app.custom-gas-input	0%	0%	❌
src.app.depth-chart	0%	0%	❌
src.app.dive-overview	0%	0%	❌
src.app.dive-plan	0%	0%	❌
src.app.dive-planner-service	76%	72%	✔
src.app.dive-settings	0%	0%	❌
src.app.dive-summary	0%	0%	❌
src.app.end-chart	0%	0%	❌
src.app.error-list	0%	0%	❌
src.app.graph-dialog	0%	0%	❌
src.app.home	0%	0%	❌
src.app.maintain-depth	0%	0%	❌
src.app.new-depth-stats	0%	0%	❌
src.app.new-dive	0%	0%	❌
src.app.new-gas-input	0%	0%	❌
src.app.new-gas-stats	0%	0%	❌
src.app.new-time-stats	0%	0%	❌
src.app.pipes	29%	15%	❌
src.app.po2-chart	0%	0%	❌
src.app.standard-gas-list	0%	0%	❌
src.app.start-gas-stats	0%	0%	❌
src.app.tissues-ceiling-chart	0%	0%	❌
src.app.tissues-phe-chart	0%	0%	❌
src.app.tissues-pn2-chart	0%	0%	❌
src.app.utility	100%	100%	✔
Summary	30% (394 / 1308)	7% (57 / 792)	❌