broker: remove dead IdempotencyStore (post-issue-#72)

[hanwencheng]

Summary

Issue #72 / PR #96 deleted POST /v1/mint-aws-creds and the crates/agentkeys-broker-server/src/handlers/mint.rs handler, which was the only production consumer of IdempotencyStore. The store remained wired through boot.rs → AppState but no live code path read or wrote through it.

What landed

• Deleted crates/agentkeys-broker-server/src/storage/idempotency.rs (the store, IdempotencyOutcome, the 4 unit tests).
• Removed pub mod idempotency; + pub use idempotency::{IdempotencyOutcome, IdempotencyStore}; from storage/mod.rs.
• Removed idempotency_store field + doc comment from AppState (state.rs).
• Removed idempotency_store field from BootArtifacts, the IdempotencyStore::open(...) block, the Ok(BootArtifacts { … }) assignment, and the idempotency_path() helper from boot.rs.
• Removed idempotency_store: boot_artifacts.idempotency_store, from the AppState constructor in main.rs.
• Removed the dead idempotency_hits + idempotency_conflicts AtomicU64 counters and their /metrics array entries from metrics.rs; no live code path bumped them. Updated the help/type-line-count test assertion from 10 → 8.
• Cleaned up IdempotencyStore::open_in_memory() boilerplate in the six integration tests that constructed AppState: auth_wallet_flow.rs, email_flow.rs, grant_flow.rs, oauth2_flow.rs, oidc_flow.rs, wallet_flow.rs.
• Removed the dedicated Idempotency-Key subsection + the agentkeys_broker_idempotency_hits_total / _conflicts_total bullet from docs/operator-runbook-stage7.md and the ## 12. Metrics + idempotency section + ### 12.2 Idempotency-Key subsection from docs/stage7-demo-and-verification.md. The BROKER_REQUEST_BODY_LIMIT_BYTES paragraph was preserved (it documents an unrelated router-level cap, not the Idempotency-Key feature).

What did NOT land

The two remaining Idempotency-Key mentions in operator-runbook-stage7.md:391 + line 407 are inside /v1/mint-aws-creds endpoint documentation that PR #96 already made stale (the endpoint itself was deleted). Cleaning those up is part of the broader /v1/mint-aws-creds-doc-residue cleanup, not specific to IdempotencyStore, and belongs to a separate follow-up PR.

No BROKER_IDEMPOTENCY_* env vars existed in env.rs (audit returned zero hits), so none to remove there.

Verification

cargo build -p agentkeys-broker-server                             # exit 0
cargo test  -p agentkeys-broker-server                             # exit 0
cargo clippy -p agentkeys-broker-server -- -D warnings             # exit 0

Post-edit audit confirms no IdempotencyStore/idempotency_store references remain in crates/agentkeys-broker-server/src/ or crates/agentkeys-broker-server/tests/.

Test plan

• cargo build passes
• cargo test passes (all integration tests)
• cargo clippy clean with -D warnings
• Reviewer confirms no live caller is reintroduced by an in-flight PR before merge