fix(cli): support multiple --header flags in ARGOCD_OPTS env var

[superShen0916]

Checklist:

• (b) this is a bug fix
• The title of the PR states what changed and the related issues number
• The title of the PR conforms to the PR title guidelines: fix(cli): support multiple --header flags in ARGOCD_OPTS env var
• I've included "Fixes ARGOCD_OPTS does not accept --header parameter multiple times #24065" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them. (N/A - CLI-only bug fix)
• Does this PR require documentation updates? No
• I have signed off all my commits as required by DCO
• I have written unit tests for my change
• My build is green
• I have added a brief description of why this PR is necessary and/or what this PR solves.

Why this PR is necessary

ARGOCD_OPTS environment variable does not support multiple --header flags, unlike the CLI. This inconsistency means users who rely on ARGOCD_OPTS (e.g., in CI/CD pipelines or shared environments) cannot set multiple headers without using a comma-separated workaround.

What this PR solves

Fixes #24065. Changes flags storage from map[string]string to map[string][]string to support repeated string slice flags (like --header) in ARGOCD_OPTS. The fix is backwards compatible with existing comma-separated values and all other flag types (bool, int, string).

Changes

• util/config/env.go: Changed flags to map[string][]string, updated LoadFlags/GetFlag/GetIntFlag/GetStringSliceFlag
• util/config/env_test.go: Added 4 new test cases for multi-value flags

Test plan

• All 24 existing tests pass
• 4 new tests: multi-value --header, mixed with bool flags, backwards-compatible CSV, single value

[bunnyshell]

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argocd-ttyd	https://argocd-web-cli-lea3ah.bunnyenv.com/

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[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 63.94%. Comparing base (5ec0603) to head (f95d6a9).

Files with missing lines	Patch %	Lines
util/config/env.go	80.00%	1 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #27768      +/-   ##
==========================================
- Coverage   63.96%   63.94%   -0.02%     
==========================================
  Files         421      421              
  Lines       57774    57775       +1     
==========================================
- Hits        36954    36944      -10     
- Misses      17339    17351      +12     
+ Partials     3481     3480       -1     

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