fix: validate feature flag endpoint management

[kongwen686]

Summary

• accept the issue-specified raw string-array body for adding feature flag endpoints while preserving the existing object wrapper format
• validate endpoint paths, reject empty or malformed paths, and reject duplicate paths before writing
• verify endpoint ownership before updates and cover duplicate/update edge cases with handler tests

Why

The endpoint management API existed, but it could silently skip duplicate adds, accept invalid paths, and update an endpoint without first proving it belongs to the feature flag in the route. These gaps make the API behavior differ from the validation and ownership requirements in #2249.

Validation

• git diff --check
• go test ./handlers -run FeatureFlagEndpoint could not run in this environment because go is not installed
• gofmt -w handlers/feature_flags.go handlers/feature_flags_test.go could not run in this environment because gofmt is not installed

Payment/contact

• Contact: chaoqiang.tian@gmail.com
• PayPal: hooqee@126.com
• EVM: 0x225b4bC64E051827c069312f82fA590Ec9632209

Refs #2249

[kongwen686]

Payment / contact note if this PR is accepted for a bounty or reward:

I am based in mainland China and Stripe onboarding may not be available for me. PayPal would be the preferred payout method if supported: hooqee@126.com.

Contact email: chaoqiang.tian@gmail.com
EVM address: 0x225b4bC64E051827c069312f82fA590Ec9632209
Stellar address: GDYPL56JCCHD75ZB2I5Z4WEWZFRD24LEC62XQ2MBL34653XKO65LA
RTC/ClawRTC address: RTC02d8536941d636aee4012596afbcd8185e9f8283
Alipay: chaoqiang457963@126.com

Please let me know which payout route is supported.