fix(env): fix os.getenv prefix collision bug in init_by_lua phase

[currenjin]

Problem

In OpenResty's init_by_lua phase, os.getenv() uses nginx's internal environment mechanism which performs prefix-based matching when env NAME=VALUE directives share a common prefix. This causes os.getenv() to return the shorter-named variable's value when the longer-named one is requested.

Example: with both KUBERNETES_CLIENT_TOKEN and KUBERNETES_CLIENT_TOKEN_FILE defined as env NAME=VALUE; directives, calling os.getenv("KUBERNETES_CLIENT_TOKEN_FILE") returns KUBERNETES_CLIENT_TOKEN's value when the shorter name appears first in the config.

Fixes #13055

Root cause

OpenResty's os.getenv in init_by_lua does not perform exact-match lookup when env NAME=VALUE; directives have a common prefix — the shorter name is matched first. In contrast, reading ffi.C.environ directly with Lua string operations (as _M.init() already does) produces a correctly keyed table.

Fix

1. apisix/core/env.lua: After _M.init() populates apisix_env_vars from ffi.C.environ, override os.getenv with a wrapper that does exact table lookup. Falls back to the original os.getenv for variables set dynamically after startup (e.g., via core.os.setenv).

2. apisix/init.lua: Move core.env.init() to be the first call in http_init(), ensuring the override is applied before any other code in init_by_lua can call os.getenv.

Tests

• TEST 10: Verifies os.getenv returns correct values when shorter-prefix variable is declared first in nginx config.
• TEST 11: Verifies os.getenv called inside init_by_lua (via extra_init_by_lua) returns correct values for prefix-colliding variables.

Risk / Compatibility

• os.getenv is monkey-patched globally after _M.init(). Existing callers continue to work — the semantics are identical for non-colliding variable names.
• Variables set dynamically via core.os.setenv fall back to the original os.getenv, so those are unaffected.
• The ordering change in http_init is safe: core.env.init() has no dependencies on resolver or id initialization.