Producers CI red on main: hash-lock drift after PyPI yanked build 1.5.1

## Problem

**Producers CI fails on `main` since `d7b41e3`** (the 0.2.0 release commit) — job "Hash-lock drift check", step "Locks must match the reviewed files (all three)". Upstream run at 2026-07-15T08:47Z failed; fork mirrors fail identically on sync, so this is deterministic, not environmental.

## Root cause

The checked-in locks pin `build==1.5.1`, but PyPI has **yanked** that release (yank reason: *"Considers breaking changes, will discuss re-releasing as a new major version"* — verify at https://pypi.org/pypi/build/json). A fresh `regen-locks.sh` run therefore resolves `build==1.5.0`, and the drift gate correctly reports that the checked-in locks no longer match a clean regeneration:

```text
-build==1.5.1 \
+build==1.5.0 \
```

(The drift check is doing its job — the locks reference an artifact pip will no longer select.)

## Suggested fix

Run `bash deploy/otlp_receiver/regen-locks.sh`, review the diff (expected: `build` 1.5.1 → 1.5.0 with hash updates in `pip-tools.lock` / `requirements.lock` / `producers/build-requirements.lock`), and commit the regenerated locks. Optionally add `build!=1.5.1` to `pip-tools.in` as a guard in case the yank is reverted, and drop it when build ships its next release.

Not related to PR #351 (touches only `examples/`, `scripts/`, `tests/`; its checks passed on head `a7de311`).

Start by running bash deploy/otlp_receiver/regen-locks.sh and review the resulting diff. Check pip-tools.lock, requirements.lock, and producers/build-requirements.lock for the expected build 1.5.1 to 1.5.0 changes and hash updates; done means the regenerated locks are committed and the Hash-lock drift check passes.

Written by the indexing model from the issue text.