chore: add e2e lambda fixture

[ava-silver]

Stack 2 of 3 -- #708 shared helpers → fixture → #710 lifecycle test + workflow.

What does this PR do?

Adds the Serverless Framework fixture the e2e suite deploys, under e2e/fixtures/lambda-node/:

• serverless.yml -- a single Node.js function plus the `serverless-plugin-datadog` config under test (layers + extension + tracing + logs). Everything unique per run (service name, freshness tag, region, keys) is injected via env so the file stays static; `frameworkVersion` and runtime are pinned.
• handler.js -- a trivial handler that emits one log line per invocation; no tracer setup, since the plugin auto-instruments.
• package.json -- pins Serverless Framework v3. The plugin itself is installed from a packed tarball at setup time (see #710).

Motivation

Infra-only slice of the e2e stack -- the workload the lifecycle test drives through APPLY/REMOVE. Kept separate so it can be eyeballed without the test logic.

Testing Guidelines

Deployed and torn down by the lifecycle test in #710; not exercised on its own.

Additional Notes

Default region is `ap-northeast-3`, overridable via `AWS_REGION`.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog

[ava-silver]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore: add e2e lifecycle test and workflow #710
• chore: add e2e lambda fixture #709 👈 (View in Graphite)
• chore: add e2e synced shared helpers #708
• main

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.69%. Comparing base (3307f10) to head (0c280ce).

Additional details and impacted files

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##           ava.silver/chore/add-e2e-shared-helpers     #709   +/-   ##
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  Lines                                         1112     1112           
  Branches                                       350      350           
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  Hits                                           864      864           
  Misses                                         118      118           
  Partials                                       130      130           

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