fix(helm): harden default container security context

[WoahitsJoe]

Description

Closes #6852

Hardens the Helm chart's default Kargo container security context by:

• requiring a non-root user
• disabling privilege escalation
• dropping all Linux capabilities
• applying the runtime default seccomp profile

Component-level securityContext values continue to override the global defaults. This also updates the generated chart documentation and adds a Helm unit test proving that the API container inherits the hardened defaults.

Checklist

Eligibility

• Linked to an existing issue with no blocking labels (kind/proposal, needs discussion, needs research, maintainer only, area/security, size/large, size/x-large, size/xx-large).
• Changes documentation only.
• Changes ten lines or fewer.

Quality

• Adds or updates corresponding tests.
• Adds or updates corresponding documentation.

AI Use Disclosure

This PR was written:

• By a human without AI assistance.
• By a human with AI assistance. A human has reviewed every line prior to opening the PR.
• By an AI with human supervision. A human has reviewed every line prior to opening the PR.
• Entirely by an AI. No human has reviewed this prior to opening the PR.

Sign-Off

All commits:

• Are signed off by their author (git commit -s) (required)
• Are cryptographically signed (git commit -S) (encouraged)

Validation

• make test-chart — 153 tests passed
• make lint-charts — chart lint passed
• generated charts/kargo/README.md with the repository's documented generator

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 31.13%. Comparing base (af85cd1) to head (efbee19).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6853   +/-   ##
=======================================
  Coverage   31.13%   31.13%           
=======================================
  Files         832      832           
  Lines       81235    81235           
=======================================
  Hits        25294    25294           
  Misses      54470    54470           
  Partials     1471     1471           

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