fix(fetchers): bound HN timestamp formatting

[chaliy]

Motivation

• The Hacker News fetcher deserializes time: Option<u64> and passed unchecked values into a year-subtraction formatter, which enables a CPU-bound denial-of-service for crafted large timestamps.
• The change aims to prevent unbounded work by rejecting out-of-range timestamps before running the custom formatter.

Description

• Added MAX_UNIX_TIMESTAMP (3000-01-01T00:00:00Z) as an upper bound for accepted Unix timestamps.
• Introduced format_unix_timestamp_bounded(ts) -> Option<String> which returns None for timestamps above the bound and otherwise delegates to the existing format_unix_timestamp.
• Updated format_hn_response() to render Time metadata only when the bounded formatter returns Some(String).
• Added test_format_unix_timestamp_bounded unit test to verify valid timestamps are formatted and extreme values (e.g., u64::MAX) are rejected.

Testing

• Ran formatting: cargo fmt --all which completed successfully.
• Ran targeted unit tests: cargo test -p fetchkit hackernews -- --nocapture, and the Hacker News tests passed (all relevant tests, including the new bounded-timestamp test, succeeded).

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Codex Task