feat(wrapper): track pending (not-yet-compounded) rewards (#718)

[ASIDISG]

Summary

Closes #718.

Adds a PendingRewards storage slot to the wrapper (vault) contract, per the epic's storage-bucket scope: "Store pending_rewards. Sync variable after fee wrappers distribute yield."

• DataKey::PendingRewards (contracts/wrapper/src/lib.rs): instance-storage i128, defaulting to 0 when never written, mirroring the existing Supply slot's read/write helper pattern.
• Synced inside distribute_rewards(): after the reward transfer completes, pending_rewards is incremented by the distributed amount via checked_add. This is a running total tracking cumulative not-yet-compounded rewards, kept separate from total_assets/calculate_share_price (which already reflect the transfer immediately via the underlying token balance, unchanged by this PR) — it exists for whatever compounding step the epic adds in a later issue. Nothing in this PR consumes or resets it.
• pending_rewards() getter: mirrors total_assets()/supply()'s existing style (plain i128, panics via ensure_initialized when uninitialized).
• Overflow safety: the checked_add releases the reentrancy lock before returning InvalidAmount on overflow — matching the early-return-after-acquire_lock() pattern already used by wrap()/unwrap()/withdraw() in this file, so a guard firing after the lock is held can never leave it stuck.
• TypeScript SDK: added a matching getPendingRewards() method to WrapperClient for parity with getTotalAssets()/calculateSharePrice().

Acceptance criteria (from #718)

• pending_rewards stored and synced after distribute_rewards
• Code compiles and follows the file's existing Soroban/Rust/TypeScript conventions
• Unit tests cover happy paths and error states

Test plan

Added 7 Rust unit tests in contracts/wrapper/src/test.rs:

• starts at 0 before any distribution
• syncs to the distributed amount after one distribute_rewards call
• accumulates correctly across multiple distributions
• unaffected by wrap/unwrap/withdraw (only reward distributions move it)
• not synced when distribute_rewards is rejected (invalid amount, or paused) — and confirms the reentrancy lock isn't left stuck by either rejected call, by successfully distributing again afterward
• uninitialized contract fails
• extended the existing test_uninitialized_access_panics to also cover pending_rewards()

Plus a TypeScript SDK surface test confirming getPendingRewards is exposed on WrapperClient.

Verified locally:

• cargo check -p bc-forge-wrapper --tests — clean
• cargo fmt -p bc-forge-wrapper -- --check — clean
• cargo clippy -p bc-forge-wrapper --all-targets --all-features -- -D warnings — clean
• npm test (sdk) — 23/23 passing
• npx eslint on the two changed SDK files — no new issues (the same two pre-existing issues from the [Math] Pro-rata reward calculation #727 PR — a CRLF/prettier diff in untouched withdraw() and an unused keypair var in an untouched test — are still present and still predate this change)

Note: this dev machine's native Rust linker is broken for both the GNU and MSVC toolchains (confirmed unrelated to any of these changes — it also fails on the pre-existing, untouched bc-forge-rate-limit crate), so cargo test couldn't be run to completion locally; cargo check --tests type-checks the full test module against the real Soroban SDK, and CI runs on ubuntu-latest with a working linker.

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[p3ris0n]

@ASIDISG please resolve conflicts