test: replace the stateful mining RPC tests with regtest coverage

[oxarbitrage]

Motivation

Over the last 40 main runs, rpc-submit-block failed 64% of the time and
rpc-get-block-template 46% — every failure I sampled a one hour timeout with zero panics and zero
assertion failures. Both sync a cached mainnet state to the tip before testing anything, so they
die in that prologue on Transaction(TransparentInputNotFound) (#11168) and never reach their own
assertions. They test a sync bug that already has an issue, not the mining RPCs.

Implements Track A of #9941.

Solution

Two commits adding coverage, then two removing what they replace.

Added — unit tests for LongPollId::submit_old(), which had none, and
getblocktemplate_long_poll_returns_submit_old_false_on_new_tip on Regtest: start a long poll,
advance the tip with generate, require a prompt submit_old: false and a template that validates
in proposal mode.

Removed — stateful::rpc::rpc_submit_block and stateful::rpc::rpc_get_block_template, with
their GCP jobs.

submitblock is already better covered on Regtest, on every PR, in seconds: 200 submissions with
typed Accepted in submit_blocks_test, plus typed Rejected assertions in
nu6_funding_streams_and_coinbase_balance that the deleted test never made. getblocktemplate's
only unique coverage was long-poll invalidation, and the deleted test never asserted it — it
pushed to a channel if let Some(false) = ...submit_old() and passed anyway when that never fired.
So this is the first real assertion of that path, triggered deterministically.

Two deliberate trade-offs: no GCP job covers mining RPCs after this (intended; ten others still gate
integration-tests-success), and Regtest disables PoW and submits coinbase-only blocks, which
reaches no code inside submit_block. Residual gap in Follow-up Work.

Tests

zebra-rpc --lib 91/91, clippy and fmt clean, new Regtest test stable over 4 runs at ~25s, all four
commits compile standalone. Mutating submit_old() to always return true fails the new tests —
the deleted test would still have passed. No changelog needed for a test: PR.

Specifications & References

#9941 (Track A plan), #11168 (the sync bug these jobs actually failed on), and BIP 22 long polling,
origin of submitold: https://en.bitcoin.it/wiki/BIP_0022#Optional:_Long_Polling

Follow-up Work

• Assert Accepted for a transaction-bearing Regtest block; everything needed is in tree, and
  Track B needs the same helper.
• Track B stays gated on feat(rpc)!: add lightwalletd-compatible gRPC server implementing CompactTxStreamer #10953, tracked by Serve light clients directly: implement the lightwalletd CompactTxStreamer gRPC interface in Zebra #11158.

AI Disclosure

• No AI tools were used in this PR
• AI tools were used: Claude Code, for the CI failure analysis and drafting the tests and this
  description. Findings were verified against the code and CI logs.

PR Checklist

• The PR title follows conventional commits format: type(scope): description
• The PR follows the contribution guidelines.
• This change was discussed in an issue or with the team beforehand.
• The solution is tested.
• The documentation and changelogs are up to date.

[v12-auditor]

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