feat(core): expose registered schemes via OperatorRegistry::schemes

[chitralverma]

Which issue does this PR close?

Closes #7904.

Rationale for this change

OperatorRegistry already stores every registered scheme in its internal factories: Mutex<HashMap<String, OperatorFactory>>, but there was no public way to read those keys back. For the global registry, that set is exactly what Operator::from_uri can construct, and it is otherwise unobservable at runtime (it is fully determined by the compiled-in services-* features).

Without it, a common mistake — a wrong dialect (s3a://, gs:// vs gcs://) or a missing services-* feature — yields a bare scheme is not registered error with no hint about which schemes are available.

What changes are included in this PR?

• Add OperatorRegistry::schemes() -> HashSet<String>, a thin read over the existing factories map under the current lock.
• Enrich the from_uri scheme is not registered error with an available context that lists the registered schemes (sorted), turning a bare error into an actionable hint.
• Add unit tests covering the accessor and the enriched error, plus a doctest on schemes().

Output (py bindings)

>>> import opendal
>>> opendal.Operator("asd",)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
opendal.exceptions.Unsupported: Unsupported (permanent) at  => scheme is not registered

Context:
   scheme: asd
   available: azblob, azdls, cos, cosn, file, fs, gcs, ghac, http, https, ipmfs, memory, obs, oss, s3, webdav, webhdfs

Are there any user-facing changes?

Yes, additive only:

• New public method OperatorRegistry::schemes().
• The from_uri unsupported-scheme error now carries an additional available context field listing registered schemes. No existing field or error kind changes; not a breaking change.

AI Usage Statement

This PR was prepared with the assistance of an AI coding agent (OpenCode, using an Anthropic Claude model) for implementation and drafting. All changes were reviewed by a human before submission.

[Copilot]

Pull request overview

Adds a public accessor on OperatorRegistry to expose which URI schemes are registered at runtime, and enriches unsupported-scheme errors from load()/from_uri with an “available schemes” hint to make misconfigurations (wrong dialect / missing services-* feature) easier to diagnose.

Changes:

• Introduces OperatorRegistry::schemes() -> HashSet<String> (with doctest) to expose registered schemes.
• Enriches the unsupported-scheme error context with a sorted list of available schemes.
• Adds unit tests for the accessor and for the enriched error output.

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[erickguan]

The scheme availability is solid and thanks to bring PR this quick. I'll jump back to issues to clarify design a bit in details.

[Xuanwo]

Good idea!