refactor: make tipset_by_height and load_child_tipset return anyhow::Result<Option<Tipset>> to distinguish error from miss

[hanabi1224]

Summary

Currently, tipset_by_height and load_child_tipset return anyhow::Result<Tipset>,
which means callers cannot distinguish between a genuine "not found" case (e.g., ts is
the current head and has no child yet) and a real storage/blockstore/IO error.

This makes it necessary to call .ok() or unwrap_or(false) at call sites to handle
the "not found" case, which silently swallows genuine errors and hurts the fast-fail
behavior expected by eth_getLogs and related paths.

Proposed Change

Refactor both functions to return anyhow::Result<Option<Tipset>>:

• tipset_by_height → anyhow::Result<Option<Tipset>>
• load_child_tipset → anyhow::Result<Option<Tipset>>

This allows callers to:

• Treat Ok(None) as a clean cache miss (e.g., tipset is the current head)
• Propagate or handle Err(_) as a genuine storage/IO failure, enabling fast-fail
  behavior in hot paths like eth_getLogs

Affected Call Sites

• src/state_manager/mod.rs — load_executed_tipset_without_events,
  load_executed_tipset, and try_lookup_state_from_next_tipset
  (around lines 467–468, 482–483, 1764–1768)
• Any other callers of the two functions

Context

Raised during review of #6750 (fix: optimize collect_events in Eth RPC).
See comment: #6750 (comment)