feat: improve height target calculation for watchdog canister

[maksymar]

This PR makes calculation of height target for watchdog canister more strict.

• Before: The target height is the median of explorer heights if there are enough explorers (>= min_explorers).
• After: The target height is the median only if enough explorers (>= min_explorers) are within a defined range of the median.

Configs (currently in prod):

• mainnet -- explorers 6, min_explorers 3, blocks range [-2, 2]
• testnet -- explorers 3, min_explorers 2, blocks range [-1000, 1000]

Making the height target calculation more strict for the testnet is necessary because the testnet has fewer explorers and greater instability, leading to large block height divergences. A stricter calculation helps reduce noise and false positives by ensuring only explorers with closely aligned heights are considered valid, improving accuracy and stability.

Mainnet explorers:

Testnet explorers:

[github-actions]

canbench 🏋 (dir: .)

No significant performance changes detected ✅

./canbench_results.yml is up to date ✅

~/work/bitcoin-canister/bitcoin-canister/scripts ~/work/bitcoin-canister/bitcoin-canister
~/work/bitcoin-canister/bitcoin-canister

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Benchmark: insert_300_blocks
  total:
    instructions: 561.73 M (-0.01%) (change within noise threshold)
    heap_increase: 6 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: get_metrics
  total:
    instructions: 87.01 M (0.01%) (change within noise threshold)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers
  total:
    instructions: 3.90 B (0.00%) (change within noise threshold)
    heap_increase: 2 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: insert_block_headers_multiple_times
  total:
    instructions: 13.90 B (0.02%) (change within noise threshold)
    heap_increase: 7 pages (no change)
    stable_memory_increase: 0 pages (no change)

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Benchmark: pre_upgrade_with_many_unstable_blocks
  total:
    instructions: 5.82 B (0.21%) (change within noise threshold)
    heap_increase: 4097 pages (no change)
    stable_memory_increase: 1792 pages (no change)

  serialize_blocktree (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_flatten (scope):
    instructions: 201.60 K (no change)
    heap_increase: 0 pages (no change)
    stable_memory_increase: 0 pages (no change)

  serialize_blocktree_serialize_seq (scope):
    instructions: 2.39 B (0.45%) (change within noise threshold)
    heap_increase: 2048 pages (no change)
    stable_memory_increase: 0 pages (no change)

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