👌 sphinx-codelinks: three `[codelinks]` reader policies to settle — unknown keys (both readers), a TOML-set `config_from_toml`, the schema error channel

[chrisjsewell]

Three reader-policy questions that #2005 (sphinx-codelinks reads ubproject.toml through ub-project) measured and deliberately left as they were, because each is a behaviour decision of its own. One issue so they are decided together; they share the [codelinks] reader.

1. Unknown keys inside [codelinks]: the two readers disagree. The Sphinx extension skips a key it does not model silently (set_config_to_sphinx's allow-list); codelinks analyse refuses the same file (CodeLinksConfig.__init__() got an unexpected keyword argument 'bogus_key', rc 2). Under projects.<name> it is the other way round: the extension's jsonschema refuses an unknown key (additionalProperties: False), the CLI ignores it. sphinx-mounts and sphinx-test-reports both WARN with a typed, suppressible subtype (mounts.unknown_key, test_reports.unknown_key); sphinx-needs skips silently. Proposal: both codelinks readers warn (codelinks.config, the subtype #2005 introduced), never refuse — a key several tool versions read at once is the case the shared file exists for. Measured cost: the docs' own docs/ubproject.toml carries no unknown key, so docs-codelinks -nW is unaffected.

2. config_from_toml is a key the TOML itself may set. It is in CodeLinksConfig.field_names(), so [codelinks] config_from_toml = "sub/x.toml" rewrites src_trace_config_from_toml after the load — which moves the use-site anchor (confdir / Path(config_from_toml).parent) to confdir/sub WITHOUT reading sub/x.toml. A file naming its own path is meaningless; the observable effect is a silently moved anchor. Proposal: the reader skips config_from_toml (and the docs say the key is conf.py / -D only). #2005 pinned today's behaviour with a test (test_toml_set_config_from_toml_moves_the_anchor in tests/test_src_trace.py) so this change flips a fenced thing.

3. check_sphinx_configuration reports a schema error as a bare raise Exception, which Sphinx renders as an ExtensionError with its crash-report banner ("please open an issue at sphinx-doc…") for what is a user configuration error (set_local_url = "yes"; an unknown key under a project). A sphinx.errors.ConfigError (or ExtensionError raised with a clean message) is the small fix; the message also says "in filed 'set_local_url'" (config.py, check_schema).

All three measured on master at d0edd0e9 by the #2005 recon (sphinx-build -E -q -b html and codelinks analyse over constructed files); none changed by #2005.