fix(rpc): two-slot coinbase cache prevents eviction between fake and real coinbase

[oxarbitrage]

Summary

• Each getblocktemplate call needs two cached coinbases at the same height: a zero-fee fake one for ZIP-317 weight estimation and the real one with actual fees
• The single-slot CoinbaseCache evicts one when storing the other, so zebrad rebuilds both shielded proofs (Sapling/Orchard) on every short-poll when the mempool has fee-paying transactions
• Replaces the single Option slot with a HashMap keyed on (height, fee) so both entries coexist, eliminating redundant proof work
• Entries from previous heights are evicted on insert, and the map is capped at 4 entries to prevent unbounded growth from fee churn

Closes #10907

@Zk-nd3r could you review/test this when you get a chance?

[Copilot]

Pull request overview

Updates the RPC coinbase cache to retain fake and real-fee coinbases simultaneously, avoiding repeated shielded proof generation.

Changes:

• Replaces the single cache slot with a bounded HashMap.
• Adds coexistence and height-eviction tests.
• No consensus or serialization impact.

The cache-cap logic has an off-by-one eviction issue, and test descriptions remain outdated. The PR description also lacks test-execution evidence.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
zebra-rpc/src/methods/types/get_block_template.rs	Implements the multi-entry coinbase cache.
zebra-rpc/src/methods/types/get_block_template/tests.rs	Tests dual entries and height eviction.

[Zk-nd3r]

Reviewed and tested head 1c551e8146852ce62537829802ba4ec7ff40b06c:

cargo test -p zebra-rpc coinbase_cache_evicts_fake_coinbase_when_real_fee_stored --lib
1 passed; 0 failed; 79 filtered out

The (height, fee) map fixes #10907's core fake/real eviction path.

One edge remains: map.len() > 4 is checked before insertion, so the fifth distinct fee is retained and the sixth clears the entire map, including the zero-fee sizing coinbase. That temporarily recreates the churn.

Suggested adjustment:

• when inserting a new key at len() >= 4, preserve the zero-fee entry and evict a nonzero-fee entry;
• cover fifth/sixth-fee churn and replacement of an existing key at capacity;
• rename the new test and stale BUG comments to describe post-fix coexistence.

Hermetic unit test only. No funds, mainnet activity, or live miner calls. I'll rerun against the updated head.

[oxarbitrage]

Thanks for the review and test run! All three suggestions addressed in 9b11aa6:

• Cap logic now uses >= 4 with a contains_key guard — when evicting, we remove a nonzero-fee entry instead of clearing the map, so the zero-fee sizing coinbase always survives.
• Added coinbase_cache_preserves_zero_fee_entry_at_capacity covering the churn edge case (5 distinct fees) and in-place key replacement at capacity.
• Renamed the test and removed the stale BUG comments.