Fix duplicate hooks from promoted embedded methods

[sjh9714]

Summary

Fixes #536.

Kong walks embedded structs to discover hook methods. When a hook is promoted through multiple embedded structs, Go reflection exposes compiler-generated wrappers on each embedding level, so the same leaf AfterApply can be invoked more than once. This keeps ordinary method lookup unchanged, but callback collection skips those promoted wrappers because the embedded value itself is visited separately.

Changes

• Skip promoted embedded method wrappers when collecting hook callbacks.
• Add a regression test for an exported embedded chain with a value-receiver AfterApply, matching the reported issue shape.

Testing

• Red check on a temporary master worktree: go test ./... -run TestPromotedEmbeddedAfterApplyCalledOnce -count=1 failed with expected 1, actual 3.
• go test ./... -run 'TestPromotedEmbeddedAfterApplyCalledOnce|TestEmbeddedCallbacks|TestHooks|TestApplyCalledOnce|TestAfterApplyFiresForEnvOnlyFlag' -count=1
• go test ./...
• ./bin/hermit env -r >/tmp/kong-hermit-env && . /tmp/kong-hermit-env && command -v golangci-lint && golangci-lint --version && golangci-lint run
• . /tmp/kong-hermit-env && golangci-lint run
• git diff --check

Note: I used Codex while preparing this change, reviewed the final diff, and ran the listed checks locally.

[alecthomas]

Nice.