feat(api): add application image endpoint

[cilps]

Introduces a new API endpoint that returns a application's image.

Description

Creates a new API route and function. Adds an error_message argument to the not_found function in confighttp.cpp. Updated docs to include new endpoint.

Screenshot

Issues Fixed or Closed

Resolves https://discord.com/channels/804382334370578482/1464609795012169988

Roadmap Issues

Type of Change

• feat: New feature (non-breaking change which adds functionality)
• fix: Bug fix (non-breaking change which fixes an issue)
• docs: Documentation only changes
• style: Changes that do not affect the meaning of the code (white-space, formatting, missing semicolons, etc.)
• refactor: Code change that neither fixes a bug nor adds a feature
• perf: Code change that improves performance
• test: Adding missing tests or correcting existing tests
• build: Changes that affect the build system or external dependencies
• ci: Changes to CI configuration files and scripts
• chore: Other changes that don't modify src or test files
• revert: Reverts a previous commit
• BREAKING CHANGE: Introduces a breaking change (can be combined with any type above)

Checklist

• Code follows the style guidelines of this project
• Code has been self-reviewed
• Code has been commented, particularly in hard-to-understand areas
• Code docstring/documentation-blocks for new or existing methods/components have been added or updated
• Unit tests have been added or updated for any new or modified functionality

AI Usage

• None: No AI tools were used in creating this PR
• Light: AI provided minor assistance (formatting, simple suggestions)
• Moderate: AI helped with code generation or debugging specific parts
• Heavy: AI generated most or all of the code changes

[ReenigneArcher]

I made a few tweaks to this PR.

1. It re-uses cover image logic from process.cpp/h, so it will return the same as what is provided to Moonlight. This will be the default image if no image is specified. It also means we don't have to specifically check all the directories like the cover directory, assets directory, etc. as this logic is already defined.
2. Moves the png validation logic to the process.cpp/h, and also uses it when Moonlight requests the cover now.
3. Adds some unit tests.

[codecov]

Bundle Report

Bundle size has no change ✅

[sonarqubecloud]

Quality Gate failed

Failed conditions
2 New issues
1 Security Hotspot
2 New Code Smells (required ≤ 0)

See analysis details on SonarQube Cloud

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[codecov]

Codecov Report

❌ Patch coverage is 21.05263% with 60 lines in your changes missing coverage. Please review.
✅ Project coverage is 15.46%. Comparing base (2141917) to head (d026479).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/confighttp.cpp	0.00%	45 Missing and 10 partials ⚠️
src/process.cpp	76.19%	2 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4627      +/-   ##
==========================================
+ Coverage   15.43%   15.46%   +0.02%     
==========================================
  Files          94       94              
  Lines       19178    19235      +57     
  Branches     8861     8898      +37     
==========================================
+ Hits         2960     2974      +14     
+ Misses      14429    14393      -36     
- Partials     1789     1868      +79     

Flag	Coverage Δ
Archlinux	11.62% <20.00%> (+0.14%)	⬆️
FreeBSD-14.3-aarch64	?
FreeBSD-14.3-amd64	13.75% <9.58%> (+0.02%)	⬆️
Homebrew-ubuntu-22.04	13.93% <20.00%> (+0.14%)	⬆️
Linux-AppImage	11.94% <20.00%> (+0.15%)	⬆️
Windows-AMD64	13.50% <20.00%> (+0.14%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
src/process.h	22.22% <ø> (ø)
src/process.cpp	7.90% <76.19%> (+6.61%)	⬆️
src/confighttp.cpp	0.00% <0.00%> (ø)

... and 40 files with indirect coverage changes

[cilps]

Why did you add the check_content_type function in the getCover function? It checks for the Content-Type application/json header, which is weird because you don't have to send any JSON in the request body to use this endpoint.

[ReenigneArcher]

I'm fixing it in #4631