[CI] Example modules are never tested, and make build ignores internal/ changes

[SebastienMelki]

Two small infrastructure gaps that cost real verification signal. Both surfaced while reviewing #212.

1. Example modules are never tested in CI

examples/*/ are separate Go modules. scripts/run_tests.sh and .github/workflows/ci.yml only build and test the root module, so nothing in examples/ is compiled or run on a PR.

Today that means these test files never execute:

• examples/market-data-unwrap/discard_unknown_test.go
• examples/market-data-unwrap/unwrap_shapes_test.go
• examples/market-data-unwrap/unwrap_quick_check_test.go
• examples/enum-encoding/…_test.go

The market-data-unwrap trio (~260 lines added in #212) is the end-to-end proof that DiscardUnknownFields reaches unwrap responses at runtime — the strongest evidence in that PR, and it runs nowhere.

There is also no drift check: examples ship checked-in generated code that no job regenerates or diffs. #212 had to absorb ~300 lines of unrelated regen churn from features merged in earlier PRs (marshalOpts threading, Accept-based content negotiation, body-first binding) purely because the example had been stale for months.

Fix: add a CI step that walks example modules, plus optionally a regen-and-diff step.

for d in examples/*/; do
  [ -f "$d/go.mod" ] || continue
  (cd "$d" && go build ./... && go test ./...) || exit 1
done

2. make build does not rebuild when internal/ changes

Makefile:51:

$(BIN_DIR)/%: $(CMD_DIR)/%/*.go | $(BIN_DIR)
	@go build -o $@ ./$(CMD_DIR)/$*

The prerequisites list only cmd/<name>/*.go. Every generator lives in internal/, so editing generation logic does not invalidate the target and make build is a no-op.

This is actively misleading because the golden tests reuse a stale binary when one exists — e.g. internal/httpgen/unwrap_test.go:31:

if _, buildStatErr := os.Stat(pluginPath); os.IsNotExist(buildStatErr) {
    // only builds when the binary is entirely absent
}

Reviewing #212 locally, this produced 5 test failures against two-month-old binaries; they vanished after rm -f bin/* && make build. A contributor verifying a codegen change can get either a false failure or a false pass with no indication why. CI is unaffected (clean checkout, no bin/), which is precisely why it stays hidden.

Fix: include the generator sources in the prerequisites, e.g.

GEN_SRCS := $(shell find internal -name '*.go' -not -name '*_test.go')
$(BIN_DIR)/%: $(CMD_DIR)/%/*.go $(GEN_SRCS) | $(BIN_DIR)

Related: #212.