fix: retry repeated transient stream disconnects

[dzianisv]

Summary

• use the provider request retry budget when stream_max_retries = 0 and the stream failed due to a transient transport disconnect
• keep zero-retry behavior for non-transport stream errors while still honoring any nonzero explicit stream retry budget
• add regressions for the core retry loop and for codex exec replaying repeated incomplete SSE streams

Closes #41.

Testing

• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/Users/engineer/workspace/codex/codex-rs/target cargo test -p codex-core effective_stream_retry_budget --color never
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/Users/engineer/workspace/codex/codex-rs/target cargo test -p codex-core --test all suite::stream_no_completed::retries_replayed_stream_request_when_stream_retries_are_disabled --color never -- --exact
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/Users/engineer/workspace/codex/codex-rs/target cargo build -p codex-cli --bin codex --color never
• CARGO_INCREMENTAL=0 CARGO_TARGET_DIR=/Users/engineer/workspace/codex/codex-rs/target cargo test -p codex-core --test all suite::cli_stream::responses_mode_stream_cli_retries_replayed_stream_request_when_stream_retries_disabled --color never -- --exact

[dzianisv]

CI note: the currently failing cargo-deny, experimental Bazel macOS, and rust-ci ubuntu lint/build checks are all also failing on main.

Tracked separately:

• Investigate Bazel macOS SDK fetch 403 in experimental workflow #43 Bazel macOS SDK fetch 403 Forbidden
• Fix cargo-deny failure for tar RUSTSEC-2026-0068 #44 cargo-deny tar advisory RUSTSEC-2026-0068
• Fix rust-ci invalid action input 'version' in workflow #45 rust-ci invalid workflow input version

The retry change in this PR is covered locally by the focused codex-core unit/integration tests and the real codex exec replay test listed in the PR body.

[dzianisv]

Rebased this branch onto the latest main after merging #46 and #51.

Local validation completed:

• cargo test -p codex-core --lib effective_stream_retry_budget
• cargo test -p codex-core --test all suite::stream_no_completed::retries_replayed_stream_request_when_stream_retries_are_disabled -- --exact
• cargo test -p codex-core --test all suite::cli_stream::responses_mode_stream_cli_retries_replayed_stream_request_when_stream_retries_disabled -- --exact

Current blockers are fork-wide CI issues rather than branch-local behavior:

• Fix cargo-deny failure for tar RUSTSEC-2026-0068 #44 cargo-deny / tar RUSTSEC-2026-0068
• Fix fork-wide argument-comment lint failures in TUI files #52 argument-comment lint failures in untouched TUI files
• Investigate Bazel macOS SDK fetch 403 in experimental workflow #43 / Fix PR CI runner allocation on dzianisv/codex fork #29 Bazel experimental failures and fork runner allocation issues