fix(git): unify transient error detection

[Neonforge98]

Problem

The required cargo clippy --workspace --all-targets -- -D warnings check fails on develop and on unrelated pull requests with ambiguous glob re-exports in git_api::commands. streaming exposed a crate-visible is_transient_error while utils re-exported the canonical git::util::is_transient_error; glob-exporting both modules made the command namespace ambiguous. git::bundle also carried a third byte-for-byte copy of the same predicate.

Solution

Make git::util::is_transient_error the single implementation. The streaming and bundle paths now import that canonical helper, and the streaming tests exercise the canonical function directly. The existing public git_api::commands::is_transient_error path remains available through utils, so no caller-facing API changes.

The resulting invariant is one transient-error classifier for all Git retry paths and one unambiguous command export.

Potential risks

Future edits to the canonical classifier now affect both streaming and bundle retry behavior together; that coupling is intentional, but changes to the shared predicate should keep both consumers in mind. The current predicates were byte-for-byte identical, so this PR does not change which errors are retried.

No dependency, lockfile, persistence, schema, wire-format, or UI behavior changes are included. Cross-platform runtime behavior was not manually exercised; workspace compilation covers all local targets, and CI remains the source for runner-platform validation. Rollback is a revert of commit 10b5aada7.

Verification

• Published PR CI run 32420916073 — Frontend typecheck/lint/unit tests passed; Rust workspace clippy passed; AI attribution passed.
• node scripts/tauri/prepare-sidecars.cjs --profile debug — passed; staged the sidecar required by the Tauri build script.
• cargo test -p git --lib — passed, 134/134.
• cargo test -p git_api --lib — passed, 90/90.
• cargo check --workspace --all-targets — passed.
• cargo clippy --workspace --all-targets -- -D warnings — passed with the exact CI command and a worktree-local target directory.
• rustfmt --edition 2021 --check crates/git/src/bundle.rs crates/git-api/src/commands/streaming.rs crates/git-api/src/commands/tests/streaming_tests.rs — passed.
• Commit hook — passed scoped clippy for git and git_api.
• cargo fmt --all -- --check — not clean on the untouched develop baseline; it reports pre-existing formatting diffs outside these three changed files. The changed files pass the targeted rustfmt --check command above.
• UI screenshots are not applicable because this PR has no user-visible UI change.

Audit

Architecture audit covered all 10 layers. Compilation and warnings pass; the duplicate implementation class was swept across the Rust workspace and reduced to the canonical helper; naming and ownership now point to the shared Git utility layer. No default branches, domain leakage, wire payloads, initialization paths, or multi-field resolvers are changed, so layers 5 and 8-10 are non-applicable beyond confirming the diff does not enter those boundaries.