`/regions/replicated` reports incorrect placement scheduling state

## Bug Report

### What did you do?

Configure placement rules for a key range and query:

`GET /pd/api/v1/regions/replicated?startKey=...&endKey=...`

Exercise the following states:

- All placement rules are satisfied.
- A missing peer has a legal scheduling target.
- A legal target is temporarily unavailable.
- A missing, down, or offline peer has no legal target.
- A Region has pending peers or no leader.
- The requested key range has no Region metadata or contains an internal gap.

This is related to #5095, which previously addressed inaccurate results caused
by the waiting list, but the endpoint still cannot reliably represent placement
scheduling progress.

### What did you expect to see?

- `REPLICATED` only when RuleChecker has no remaining placement work.
- `INPROGRESS` when placement is incomplete but can still progress, is waiting
for temporary state recovery, or the requested range metadata is incomplete.
- `PENDING` only when the current topology has no legal placement action.

The status query should be side-effect free and should not require a full Store
scan for every completed Region.

### What did you see instead?

The endpoint combines placement fit results with state from the checker pending
list. The pending list represents historical or temporary filtering state, not
whether the current topology has a legal scheduling target.

As a result:

- Schedulable or temporarily blocked work can be reported as `PENDING`.
- Placement work with no legal target can remain `INPROGRESS`.
- An empty or discontinuous Region scan can be reported as complete.
- Reading the status can interact with placement-fit cache state.

### What version of PD are you using (`pd-server -V`)?

Current `master` at commit `070828c2b2032c499ff224c3f3730b5637fd3226`.

Start at the GET /pd/api/v1/regions/replicated endpoint and trace how RuleChecker results, the checker pending list, placement-fit cache state, and Region metadata determine each status. Exercise the listed topology and key-range cases, then verify that status reads are side-effect free and avoid a full Store scan for completed Regions.

Written by the indexing model from the issue text.