ci: enforce clippy -D warnings and clear the warning backlog (#972)

[tschm]

Summary

Both clippy steps in ci.yml exit 0 even when there are warnings. This PR clears the clippy backlog and adds -D warnings, so any new warning fails the build.

There are three commits, in this order:

1. The two real defects

• fed_rewriter.rs: create_sources checked manual_info.is_some() and then called unwrap(). It now uses if let Some(manual_info) = &db_info.manual_info.
• sources/trino/mod.rs: the Option<String> producer already matches all six serde_json::Value variants, so its trailing _ => throw!(..) arm could never run. I removed it. The other producers' _ arms are reachable, so they stay.

2. Remaining warnings

• clippy::result_large_err (32 sites) is allowed crate-wide in lib.rs, with the reason written next to it. Every hit comes from two third-party errors held by value, Trino's (232 B) and BigQuery's (176 B), and they propagate into ConnectorXOutError. Boxing them would change public error types, and the error path is cold.
• dispatcher.rs: ///! → //! for the module doc. prepare() now returns a named Prepared<'w, S, D> alias (type_complexity).
• constants.rs: cfg(any(feature = "src_oracle")) → cfg(feature = "src_oracle").
• mssql/tiberius_impl.rs: decode(..)?.to_owned() cloned the Cow itself, so it is now into_owned(). Two single-arm matches become if let.
• connectorx-cpp: impl From<Plan> for CXFederatedPlan replaces impl Into, and one let x = ..; x becomes a direct return. The second clippy step uses --all, which covers this crate.

3. CI
Both steps now run cargo clippy ... -- -D warnings --force-warn missing_docs.

#![warn(missing_docs)] (from #986) still reports 662 undocumented items. #986 called out that a blanket -D warnings would conflict with that. --force-warn missing_docs keeps those as visible warnings that -D warnings doesn't promote, so the docs backlog can be worked down separately. Once it's at zero, the flag can go. One side effect is that connectorx-cpp also reports 9 missing_docs warnings in the --all step. They are warnings only.

Verification

On macOS arm64 with clippy 0.1.97 (Rust 1.97):

• cargo clippy --features all -- -D warnings --force-warn missing_docs exits 0.
• cargo clippy --features all --all -- -D warnings --force-warn missing_docs exits 0.
• cargo test --lib --features all: 78 passed, 0 failed.
• cargo fmt --all --check is clean.

Not addressed

• Windows-only code (cfg(windows) in the mssql source) isn't linted by this CI, which runs on Linux. I applied the same into_owned() change there anyway for consistency.
• CI installs the stable toolchain, so a new clippy release that adds lints can turn CI red without any code change. If that turns out to be noisy, pinning the toolchain is the usual fix.
• The mssql changes touch tiberius_impl.rs, which Phase 4: remove the Tiberius MSSQL backend #962 deletes. Whichever lands second will hit a trivial conflict.

Closes #972

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 45.12%. Comparing base (118450b) to head (8c60d6e).

Files with missing lines	Patch %	Lines
connectorx/src/sources/mssql/tiberius_impl.rs	77.77%	2 Missing ⚠️
connectorx/src/fed_rewriter.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1000      +/-   ##
==========================================
- Coverage   45.18%   45.12%   -0.06%     
==========================================
  Files          65       65              
  Lines        9606     9593      -13     
==========================================
- Hits         4340     4329      -11     
+ Misses       5266     5264       -2     

Files with missing lines	Coverage Δ
connectorx/src/dispatcher.rs	68.42% <100.00%> (-2.78%)	⬇️
connectorx/src/sources/trino/mod.rs	34.21% <ø> (+0.08%)	⬆️
connectorx/src/fed_rewriter.rs	0.00% <0.00%> (ø)
connectorx/src/sources/mssql/tiberius_impl.rs	91.58% <77.77%> (ø)

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[wangxiaoying]

Hi @tschm, thanks for your efforts in maintaining this project! I’ve invited you as a collaborator and hope this makes it easier for you to contribute to ConnectorX.