refactor: consolidate DataKey::Paused into CommonDataKey

[Rafiat30]

Closes #573

Summary

Both governance_contract::DataKey and settlement_contract::DataKey declared their own Paused variant, even though both contracts already read and wrote the pause flag exclusively through bettapay_common::storage::is_paused / set_paused, which operate on bettapay_common::CommonDataKey::Paused. The local Paused variants in each contract's own DataKey enum were dead code — never constructed or matched anywhere except their own declaration — left over from before the shared helper existed. Their continued presence is exactly what the issue describes: duplicated key names, a shared-keyspace illusion, and doc ambiguity about which enum actually owns the key.

This PR removes the duplicate declarations so CommonDataKey::Paused is the single source, and documents what "storage separation" actually means for a key type shared this way (each contract's instance storage is a distinct ledger entry — sharing the enum shares only the wire shape of the key, never any data).

Changes

Modified files

• governance_contract/src/lib.rs — removed the unused Paused variant from DataKey; rewrote the comment above the enum to correctly list only RecoveryAddress/PendingRecovery/Paused as living in CommonDataKey (the previous comment incorrectly grouped Admin in with them, even though Admin has always stayed on each contract's own DataKey) and to explain storage separation.
• settlement_contract/src/types.rs — same removal and comment fix for its DataKey enum.
• bettapay_common/src/storage.rs — expanded the module doc comment: explains that CommonDataKey sharing a Rust type across contracts does not share any on-chain data (Soroban instance storage is scoped per contract address), and reworked the old SCVal-encoding-compatibility note into a historical note now that the duplicate variants it justified have been removed.

Test files

• bettapay_common/test_snapshots/storage/tests/is_paused_defaults_to_false_and_round_trips_via_common_data_key.1.json (new)
• bettapay_common/test_snapshots/storage/tests/two_contract_instances_have_independent_paused_flags.1.json (new)

No production contract behavior changed: is_paused/set_paused already read/wrote CommonDataKey::Paused before this PR; removing the dead local variants doesn't touch any storage path either contract actually exercises.

Implementation details

• Verified the local Paused variants were genuinely unreferenced (grep -rn "DataKey::Paused" across both contracts only ever matched the enum declaration itself, never a .get/.set/.has/.remove call site) before removing them.
• Kept the change scoped to Paused specifically, per the issue title. RecoveryAddress and PendingRecovery have the identical dead-variant pattern in both contracts' local DataKey enums (the existing comment already claimed they "live in CommonDataKey instead of here" even before this PR) — left untouched to keep this PR focused on a single concern per CONTRIBUTING.md; a natural follow-up if the maintainers want it.
• #[contracttype]-derived enums encode on-chain by variant name only (not by the parent enum's Rust name), which is why this consolidation needed no storage migration — documented as a historical note in storage.rs since it's still useful context for why this was safe, even though the duplicate variants that made the point concrete are now gone.

Tests added

• is_paused_defaults_to_false_and_round_trips_via_common_data_key — registers a dummy contract, then asserts is_paused reads back false with no entry written, and that set_paused(true)/set_paused(false) round-trip correctly, all against CommonDataKey::Paused directly.
• two_contract_instances_have_independent_paused_flags — registers two separate contract instances, pauses one via CommonDataKey::Paused, and asserts the other instance's is_paused is unaffected. This is the executable proof that sharing the key type across contracts does not share any pause state between them — the "document storage separation" half of the issue.

Both new tests live in bettapay_common::storage's existing #[cfg(test)] mod tests block, alongside the existing primary_admin_* tests, using the same Env::default() + testutils::Address pattern already used there (plus a minimal #[contract] struct DummyContract;, needed because Env::as_contract requires a registered contract address to switch storage context into).

How to test

# Build WASM once (governance_contract's tests embed it via include_bytes!)
cargo build --target wasm32-unknown-unknown --release

# Full workspace suite
cargo test --workspace

# Just the new/relevant tests
cargo test -p bettapay_common storage::tests

# Confirm no code references the removed variant anywhere
grep -rn "DataKey::Paused" governance_contract/src settlement_contract/src
# (only bettapay_common::CommonDataKey::Paused usages should remain)

# Clippy stays clean with the removed dead variants
cargo clippy --workspace --all-targets --all-features -- -D warnings