api: add Config Entries and Discovery Chain endpoints

[mbrulatout]

Summary

Phase 2 of the ongoing API-parity work (follow-up to #120). Adds two more endpoint groups:

• Config Entries (consul/api/config.py) — generic CRUD for /v1/config, covering all config entry kinds (service-defaults, service-router, service-splitter, service-resolver, proxy-defaults, mesh, ingress-gateway, terminating-gateway, exported-services, api-gateway, etc.) through one Kind/Name-based implementation rather than modeling every kind's schema individually. Includes cas (check-and-set) support on both set() and delete().
• Discovery Chain (consul/api/discovery_chain.py) — consul.discovery_chain.get(), covering both the plain GET and the POST variant Consul requires when passing override parameters (connect timeout, protocol, mesh gateway mode).

Not included, on purpose:

• Snapshot — its response is a raw gzip binary blob, but the existing sync/async HTTP clients (consul/std.py, consul/aio.py) unconditionally decode every response body as UTF-8 text. Implementing Snapshot correctly needs a small transport-layer change (an opt-in raw/binary response mode) that's a distinct, separate piece of work deserving its own isolated review rather than being bundled into an "add a CRUD module" PR. Will follow up separately.
• Peering — deliberately deferred; no current consumer identified for cluster peering.

A bug this surfaced

While testing Config.set()/delete() with cas, I found that PUT /v1/config always returns a literal JSON boolean (true/false) reflecting whether the CAS check passed — but DELETE /v1/config/:kind/:name returns an empty object ({}) when no cas is given, and only returns a real boolean when cas is passed. Using CB.boolean() (which only checks the HTTP status code, not body content) would have silently reported CAS failures as successes. Fixed by using CB.json() for set(), and branching between CB.boolean() / CB.json() in delete() based on whether cas was supplied. Verified against a live Consul instance before writing the test assertions.

Base branch note

This branches off master, not off #120 — the two PRs touch disjoint files except for .pre-commit-config.yaml's mypy fix, which is carried over here too (see last commit) since master doesn't have it yet. ENDPOINT_STATUS.md isn't touched in this PR since it doesn't exist on master yet (it's new in #120); I'll add Config Entries/Discovery Chain rows to it once this rebases past #120's merge.

Test plan

• mypy consul/ --no-sqlite-cache — clean
• ruff check . / ruff format --diff . — clean
• New integration tests in tests/api/test_config.py and tests/api/test_discovery_chain.py, run against real dockerized Consul (1.20.6/1.21.5/1.22.0) — all passing, including both CAS-success and CAS-failure paths on set() and delete()
• Full existing test suite re-run — no regressions