fix: omit empty Bluesky reply field

[vivanvv]

Summary

• Omit the Bluesky reply field from post records when there is no reply reference
• Add a regression test covering the com.atproto.repo.createRecord payload shape

Fixes #4.

Testing

• cargo fmt --check
• cargo test --locked
• cargo clippy --all-targets

I did not test by live-posting to Bluesky. The regression test covers the rejected payload shape by checking that normal posts no longer serialise reply: None as "reply": null.

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