feat: optimize marketplace contracts with tests and docs

[mgtmdccix-oss]

Summary
Rewrites
lib.rs
to improve security, robustness, and developer experience across the Marketplace domain. No changes to other contracts.

Changes
Security
Reentrancy guard on fund_invoice using kora_shared::reentrancy::acquire_guard. The function is split into a public entry point (holds the guard) and a private fund_invoice_inner (does the work), so the guard cleanly wraps the entire operation including token transfers and the cross-contract pool call.
Checks-effects-interactions enforced explicitly — funded_amount and is_active are written to storage before any token transfers occur.
net > 0 guard after fee calculation prevents a 100% fee misconfiguration from sending zero tokens to the pool.
require_not_paused stub added to all mutating functions (list_invoice, fund_invoice, cancel_listing), ready for access_control integration at deployment.
Robustness
TTL bumping on all persistent storage writes via bump_listing() and inline extend_ttl for whitelisted tokens — matches the pattern used in access_control and risk_registry.
Typed errors on instance reads — replaced .unwrap() on InvoiceNft, FinancingPool, and Treasury address reads with .ok_or(KoraError::NotInitialized).
Token address captured before storage write to eliminate any borrow ambiguity around listing.token.
New functions
Function Description
update_fee_bps(admin, new_fee_bps) Admin can update the fee rate post-deploy
dewhitelist_token(admin, token) Admin can remove a token from the whitelist
get_fee_bps() View the current fee rate
get_admin() View the current admin address
is_token_whitelisted(token) Check whitelist status of a token
Tests
58 unit tests added covering all functions and edge cases:

initialize — boundary fee values (0, max, over-max), double-init
get_admin / get_fee_bps — correct values, pre-init error
update_fee_bps — success, boundary values, non-admin rejection, state unchanged on failure
whitelist_token / dewhitelist_token — full lifecycle, non-admin rejection, re-whitelist after removal
is_token_whitelisted — unknown token returns false
list_invoice — success, NFT status transition, all validation rejections (zero/negative price, price ≥ face value, past deadline, non-whitelisted token, duplicate id), multiple independent listings
get_listing — not found, correct data returned
fund_invoice — partial/full funding, deadline expiry, cancelled listing, exceeds target, fee math, multiple investors, exact remaining amount, one-over-remaining, zero-fee edge case
cancel_listing — seller, admin, stranger, not found, already cancelled, state unchanged on failure, partial-funded cancel
Arithmetic safety — overflow protection on funded_amount
Checklist
Scoped to contracts/marketplace/ only
All arithmetic uses checked_* / bps_of — no silent overflows
No unwrap() in contract code
require_auth() is the first call in every mutating function
Follows TTL bumping pattern from access_control and risk_registry
Compatible with existing Soroban SDK 21.0.0 infrastructure
Existing integration tests in contracts/tests/ unaffected
closes #101

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