fix(env): resolve os.getenv prefix collision for env directives

[AlinsRan]

Fixes #13055

Problem

When two env NAME=VALUE; directives share a common prefix (e.g. KUBERNETES_CLIENT_TOKEN and KUBERNETES_CLIENT_TOKEN_FILE), resolving the longer-named variable during the init / init_worker phase returns the shorter variable's value, depending on declaration order.

Root cause

Before any request is served, lua-resty-core replaces os.getenv with a shim that delegates to the C function ngx_http_lua_ffi_get_conf_env. That function matches the queried name against each configured env entry with:

if (var[i].data[var[i].len] == '='
    && ngx_strncmp(name, var[i].data, var[i].len) == 0)

It checks that the configured entry ends at var[i].len, but it does not require the queried name to end there too. So os.getenv("KUBERNETES_CLIENT_TOKEN_FILE") matches the KUBERNETES_CLIENT_TOKEN entry on its first 23 bytes and returns the wrong value. This is an upstream lua-nginx-module bug, but it can be fully sidestepped on the APISIX side.

PR #13147 cannot fix it, because during init_by_lua the environ snapshot does not yet contain env NAME=VALUE; directive vars (nginx applies them only in ngx_set_environment at worker start), so its lookup misses and falls back to the same shim.

Fix (self-contained, no OpenResty change)

• Resolve env variables from environ directly with exact keys (core.env.init() already builds such a table) instead of going through the buggy shim.
• nginx assigns the directive vars to the global environ in ngx_set_environment at worker start (before init_worker_by_lua), so core.env.init() is also invoked in the worker init phase to capture directive-assigned values.
• Add core.env.get(name) for exact-match lookups; route $ENV:// resolution and the kubernetes discovery read_env through it.

Tests

t/core/env.t:

• TEST 10/11: prefix-colliding directives resolve correctly via $ENV://, regardless of declaration order.
• TEST 12: core.env.get returns exact values for prefix-colliding directives.
• TEST 13: values captured in the init_worker phase for prefix-colliding directives are correct.

Compatibility

core.env.get falls back to os.getenv for variables set dynamically after startup (e.g. via core.os.setenv), so existing behavior is preserved. The new worker-phase core.env.init() is idempotent.