Allow abstracting over process.env

[mbg]

Two straight-forward commits cherry-picked from #3990 (and, in turn, #3973) which add an Env interface that allows us to abstract over process.env so that tests don't have to run synchronously because they query process.env and can instead query a test-specific environment.

Not used in this PR, but cherry-picked so that we can hopefully merge it relatively quickly and make use of it elsewhere.

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• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
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Pull request overview

This PR introduces a small abstraction layer over environment access to avoid hard-coding process.env in places that need to be testable, and begins standardizing GitHub Actions environment variable names via an enum.

Changes:

• Add an Env interface and getEnv(...) helper to abstract reads from process.env (or a provided env map).
• Split env-var access helpers into *From(env, ...) variants plus process.env wrappers.
• Add ActionsEnvVars enum and update some call sites/tests to use it instead of raw strings.

Show a summary per file

File	Description
src/util.ts	Adds Env adapter and env accessor helpers that can take a NodeJS.ProcessEnv.
src/environment.ts	Defines the new Env interface used to abstract environment lookups.
src/testing-utils.ts	Adds a test helper for creating an Env and tightens typing for default Actions env vars.
src/actions-util.ts	Introduces ActionsEnvVars enum and replaces some raw env var strings with enum constants.
src/api-client.ts	Updates env-var lookups to use ActionsEnvVars constants.
lib/entry-points.js	Changed but excluded from diff (and typically generated output).

Review details

Files excluded by content exclusion policy (1)

• lib/entry-points.js

Comments suppressed due to low confidence (1)

src/util.ts:588

• Env.getRequired and Env.getOptional currently treat whitespace-only values differently: getOptionalEnvVarFrom treats " " as unset (via trim()), but getRequiredEnvVar accepts it because it only checks value.length === 0. This inconsistency is surprising for callers and can hide misconfiguration.

  const value = env[paramName];
  if (value === undefined || value.length === 0) {
    throw new Error(`${paramName} environment variable must be set`);
  }
  return value;

• Files reviewed: 5/6 changed files
• Comments generated: 3
• Review effort level: Low

[sam-robson]

LGTM 👍🏻