extract pup-owned helpers out of the generator-owned client.rs/util.rs

[MintsInc]

Context

openapi-transformer's pup generator treats client.rs and util.rs as fully generator-owned files, copied byte-for-byte from a bundled snapshot on every regeneration. Tracing what generated command modules actually reference from them (only make_api!/make_api_no_auth! and util::read_json_file) showed nearly everything else in both files was pup's own hand-written command logic that just happened to live there. That mismatch caused a real regression in pup#651: regenerating the client silently deleted an OAuth-exclusion table entry and a read_to_string helper, both added by #654 ("feat(events): add event posting") after the last snapshot sync.

Changes

Split both files along the generator/hand-written boundary, mirroring how main.rs/generated.rs already separate hand-written and generated code:

• client.rs shrinks to exactly what make_api!/make_api_no_auth! need: BearerAuthMiddleware/UserAgentMiddleware, make_dd_config/make_dd_client, the two macros, UNSTABLE_OPS.
• New src/raw_client.rs holds everything else: HttpError/HttpResponse, AuthType/get_auth_type, the OAuth-exclusion table + apply_auth, parse_response_json, and all raw_* HTTP helpers — the "raw/generic Datadog HTTP client" subsystem used by ~16 command files for endpoints not covered by the typed SDK.
• util.rs shrinks to just read_json_file. New src/util_ext.rs holds everything else: time/duration parsing, the monitor-diff helpers, parse_compute_raw, percent_encode.
• This is a pure move — identical function bodies/signatures, only import paths changed across ~30 call sites. No behavior change.
• Once this lands, these two files become safe for openapi-transformer to fully own without future feature work (like feat(events): add event posting #654) getting silently reverted on the next regeneration.

Tests

cargo build --all-targets, cargo test, cargo clippy --tests -- -D warnings, cargo fmt --check all pass. cargo test shows 3 pre-existing failures (test_cases_timeline/test_dbm_samples_search_uses_documented_payload/test_monitors_diff_detects_changes/test_spans_metrics_list, one of which varies per run) — confirmed identical on unmodified main (a PUP_MOCK_SERVER env-var race across parallel test threads), unrelated to this change.