fix(services/github): support env-driven construction, honor range reads, add read-only behavior tests

[trim21]

Which issue does this PR close?

None.

Rationale for this change

Three related gaps block the github service from having behavior test coverage and working correctly with a token:

• GithubConfig::from_uri requires owner and repo to be parsed from the URI authority, which breaks Operator::via_iter("github", ...) construction used by env-driven setups and the behavior test framework. The github service is the only service that cannot be configured through environment variables.
• GitHub ignores the Range header on authenticated Contents API requests and returns the full content with 200, so range reads (.range() / suffix reads) return too much data when a token is configured.
• The service has no behavior tests, which is why it stayed excluded from bindings.

What changes are included in this PR?

• Fix from_uri to fall back to options (OPENDAL_GITHUB_OWNER / OPENDAL_GITHUB_REPO) when the URI has no authority or path; github://<owner>/<repo>/<root> URI semantics are unchanged. Owner and repo stay validated as non-empty since #[serde(default)] would otherwise silently turn a missing field into an empty string.
• Fix range reads: when the server returns 200 (ignored range) instead of 206, slice the response client-side, matching the pattern used by etcd/sled/postgresql.
• Add .github/services/github/read_only/action.yml that runs the read-only behavior tests against the fixtures in core/tests/data of the apache/opendal repository itself, so no extra backend or credentials are required.
• Inject GITHUB_TOKEN into the core and python binding behavior test steps so the github tests can raise their API rate limit.
• Add unit tests covering option-driven construction.

Once merged, the behavior test matrix for core and all language bindings automatically picks up the github case.

Reproduction

repro_github_range.sh demonstrates the Range behavior difference (requires gh and curl):

• Anonymous request: 206 with content-range: bytes 1024-2047/30482 (1024 bytes).
• Authenticated request (gh auth token): 200 with the full 30482-byte content — the Range header is ignored.

Are there any user-facing changes?

OPENDAL_GITHUB_OWNER / OPENDAL_GITHUB_REPO environment variables now work for configuring the github service, and range reads work correctly when a token is configured.

AI Usage Statement

This PR was developed with the assistance of an AI coding agent (pi) for implementation, testing, and PR preparation; the changes were reviewed by the author.