Support Fess 15.7 unified /api/v2 health endpoint

[marevol]

Summary

Fess 15.7 unified its public API under /api/v2 and removed the legacy /api/v1 (and /api/chat) surface. The admin API (/api/admin/*) is unchanged, so the only call in fessctl affected by this is the health check used by ping, which moved from /api/v1/health to /api/v2/health and now returns the v2 response envelope.

This PR makes fessctl work correctly against any supported Fess version (14.x and 15.x), gated by FESS_VERSION.

Changes

• is_api_v2 version gate: add FessAPIClient.is_api_v2 ((major, minor) >= (15, 7)) and route ping() to /api/v2/health on 15.7+, /api/v1/health on older versions.
• v2 envelope parsing: parse the v2 health response in the ping command — cluster status is read from response.engine (success) or response.error.details.engine (red cluster, HTTP 503) — while keeping the v1 {"data": ...} handling for older servers.
• Default version: change the default FESS_VERSION from 15.6.1 to 15.7.0. Fess 14.x and 15.x remain supported through the existing version-gated behavior (CRUD HTTP method selection and the health endpoint).
• Integration tests: make the health-wait URL in conftest.py version-aware and bump the compose-fess15.yaml default image tag.
• Docs: clarify the FESS_VERSION behavior in the README.

Version behavior matrix

FESS_VERSION	CRUD HTTP methods	Health endpoint
14.x	create=PUT / edit=POST / start,stop=POST	/api/v1/health
15.0–15.6	create=POST / edit=PUT / start,stop=PUT	/api/v1/health
15.7+ (default)	same as 15.x	/api/v2/health (v2 envelope)

Tests

• Added unit tests: v2 ping endpoint/headers, is_api_v2 boundary across 14.x/15.x/16.x, default-version uses v2, v2 ping output (green/yellow/red/json), and a send_request case asserting a non-2xx JSON body (red-cluster 503) is parsed rather than raised.
• Existing v1 ping tests pin is_api_v2 = False to keep covering the pre-15.7 path.
• All 120 unit tests pass (uv run pytest tests/unit/).