Fix linting errors and re-enable golangci linters

[scottisloud]

Overview

Following up on #595, the errcheck, depguard, and staticcheck linters were previously disabled in .golangci.yml because they were producing errors after the linter had been nonfunctional for an extended period of time. This PR resolves all outstanding issues and re-enables them so every configured linter runs in CI.

Closes OS-148

Type of change

• Created a new plugin
• Improved an existing plugin
• Fixed a bug in an existing plugin
• Improved contributor utilities or experience

Changelog

Changes
depguard (1 fix)

• sdk/plugintest/example_secrets.go — swapped math/rand for the safer crypto/rand.

errcheck (4 fixes)

• sdk/plugintest/validation_report.go — checked the return values of the Printf calls on Heading, Success, Warning, and Error formatters.

staticcheck (7 fixes / suppressions)

• plugins/readme/api_key.go — dropped the redundant string type declaration (ST1023).
• sdk/plugintest/validation_report.go — removed the unnecessary nil check before len() (S1009).
• sdk/schema/executable.go — simplified a boolean expression via De Morgan's law (QF1001).
• plugins/aws/sts_provisioner.go and sdk/rpc/server/server.go — added targeted //nolint exclusions for QF1008 (embedded-field selectors) where the explicit form was preferred for readability.

.golangci.yml

• Removed the disable: block; errcheck, depguard, and staticcheck are now in the enable: list alongside the existing defaults.

Testing

golangci-lint run now exits cleanly with all three linters active.

[JillRegan]

Left a small comment that I don't have a strong opinion about so approving! 🙌