fix: Assert connection is loaded in PeerReservationTable mutators

[SAY-5]

High Level Overview of Change

PeerReservationTable::connection_ is a raw pointer initialized to nullptr and only set later by load(). Both insertOrAssign() and erase() dereference it unconditionally (connection_->checkoutDb()), so calling either before load() runs would be an unchecked null dereference. This adds an XRPL_ASSERT(connection_, ...) at the top of both mutators to make the "must be loaded first" precondition explicit, consistent with how the rest of the overlay code guards its invariants. Fixes #7509.

Context of Change

In normal startup load() is called during ApplicationImp::setup before any RPC handler can reach these methods, so this is a defensive guard rather than a fix for an observed crash. The assertion documents and enforces the ordering invariant, and turns a would-be null dereference into a clear diagnostic in debug builds.

API Impact

• Public API: New feature (new methods and/or new fields)
• Public API: Breaking change (in general, breaking changes should only impact the next api_version)
• libxrpl change (any change that may affect libxrpl or dependents of libxrpl)
• Peer protocol change (must be backward compatible or bump the peer protocol version)

Test Plan

The added assertions only fire when the invariant is violated (mutator called before load()), which the existing peer-reservation tests never do, so those tests continue to pass unchanged and no new test is needed for the guarded path. The change is formatted with the repository .clang-format.

[github-actions]

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