Add 404 non-retryable test coverage for Reddit adapter

[gdiab]

Context

Claude re-review on PR #8 called out a small test coverage gap in the Reddit adapter tests.

Reference: #8 (comment)

Problem

src/adapters/reddit-adapter.test.ts currently covers:

• successful fetch/parse
• retryable 5xx failure

It does not cover a 404 response, which should be non-retryable under the current retryability rule (status >= 500 || status === 429).

Proposed change

• Add a test case for HTTP 404 in reddit-adapter.test.ts.
• Assert that the adapter returns an ingest failure with retryable: false for 404.

Acceptance criteria

• A 404 test exists and passes.
• Test clearly verifies non-retryable behavior for 404.
• Existing adapter tests remain green.

Priority

Minor / non-blocking follow-up from PR #8.