refactor(provider): deduplicate SSE transport via shared helper

Extract shared transportStream seam behind providerRunStream/runStream
adapters, removing ~200 lines of duplicated retry/timeout/abort/
redaction/SSE-parse/cost/fallback logic. No behavior change — both
transports now delegate to a single deep module (small interface:
url + bodyStr + headers + signal + sink + eventToParts mapper; large
implementation hidden). Gives leverage (two callers) and locality
(fix once, fixed everywhere). Verified: typecheck, test:unit,
test:integration, test:contract, format:check and
provider-transport.test.ts green (16/16).

Closes #53