Clamp per-series retention overrides to server default

[ne0ekspert]

Motivation

• Prevent unauthenticated clients from bypassing operator-configured retention by setting an arbitrarily large per-series max_storage_bytes which could exhaust disk.
• Preserve existing behavior that allows clients to request stricter (smaller) per-series limits while enforcing the server-side configured default as an upper bound.

Description

• Change Engine::register_series to clamp a client-provided max_storage_bytes to default_series_max_bytes when a server default is configured, using requested_min(default) semantics and preserving None semantics when no default is set.
• Keep the existing zero-value validation for max_storage_bytes and continue to persist the chosen limit into the manifest.
• Add a regression unit test register_series_clamps_client_limit_to_default_cap that registers a series with an oversized client limit and asserts the persisted max_storage_bytes equals the configured default.

Testing

• Ran cargo fmt successfully.
• Ran cargo test register_series_clamps_client_limit_to_default_cap -- --nocapture and the test passed.
• Ran cargo test evicts_oldest_segment_when_series_limit_is_exceeded -- --nocapture and the test passed.

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Codex Task