fix(cli): skip commented-out lines when resolving env vars in apisix.yaml

[asdf2014]

Description

Standalone mode fails to boot when apisix.yaml contains a commented-out env var reference:

# ${{MY_ENV_VAR}}

Since #13078 (shipped in 3.17.0), ${{VAR}} is substituted on the raw text before YAML parsing so the parser can infer scalar types — which means comment lines are substituted too, and an unset var inside a comment aborts startup with can't find environment variable MY_ENV_VAR. (config.yaml is unaffected: it substitutes on the parsed table where comments are already gone; same for JSON standalone configs, which also resolve post-parse.)

This patch makes the raw-text substitution run line by line, keeping full-line comments (and blank / no-reference lines) verbatim, reusing the existing is_empty_yaml_line predicate. A leading UTF-8 BOM is detached first so a first-line comment is still recognized. Unset vars on data lines still fail with the same error message; output is byte-identical to the previous implementation for comment-free input (line endings, missing trailing newline, and the #END marker are all preserved). Both callers — CLI init (cli/file.lua) and the runtime hot-reload (core/config_yaml.lua) — share the fixed function.

Behavior notes (accepted trade-offs of the text-level heuristic, documented in the code comment):

• inline trailing comments (key: val # ${{VAR}}) are still substituted — # may legally appear inside YAML strings, and telling those apart requires a full parse, which can only happen after substitution;
• a block-scalar content line that itself starts with # is now skipped like a comment (no in-tree config or doc example hits this; the failure mode is a visible literal ${{VAR}}, not a crash);
• a ${{...}} reference spanning a physical line no longer matches (all in-tree references are single-line);
• a var referenced only in a comment no longer emits an env VAR; nginx directive (comments having side effects was part of the bug).

The new test in t/cli/test_standalone.sh covers both paths: make init (CLI) and make run + request (runtime hot-reload) against a config whose commented-out lines reference unset vars.

Which issue(s) this PR fixes:

Fixes #13685

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)