Follow 303 redirects when the API returns them

[delner]

Fix HTTP 303 redirect handling for large dataset responses

Fixes #101

Problem

The Braintrust API returns HTTP 303 redirects to S3 presigned URLs when responses are large (e.g., dataset fetches via POST /btql). The SDK treated all non-2xx responses as errors, so these redirects raised exceptions like:

HTTP 303 for POST https://api.braintrust.dev/btql: See Other

Solution

Extracted a new Braintrust::Internal::Http module with two composable utilities:

• Http.with_redirects(uri, request) — drop-in replacement for raw Net::HTTP request execution that follows HTTP redirects. Handles 301/302/303 (follow with GET), 307/308 (preserve method and body), strips Authorization headers on cross-host redirects (e.g., API → S3), and enforces a configurable max redirect limit.

• Http.decompress_response!(response) — decompresses response bodies based on Content-Encoding (gzip, x-gzip). No-op if uncompressed.

The following HTTP call sites now use with_redirects:

• API::Datasets — list, get, create, insert, fetch (fetch also calls decompress_response!)
• API::Functions — list, create, invoke, get, delete
• API::Internal::Projects — create
• API::Internal::Experiments — create
• API::Internal::Auth — login
• Trace::Attachment — from_url

Redirect handling

Status	Before	After
301 Moved Permanently	Raised Braintrust::Error	Follow with GET
302 Found	Raised Braintrust::Error	Follow with GET
303 See Other	Raised Braintrust::Error	Follow with GET, strip auth on cross-host
307 Temporary Redirect	Raised Braintrust::Error	Follow, preserving method and body
308 Permanent Redirect	Raised Braintrust::Error	Follow, preserving method and body
gzip Content-Encoding	Not handled	Decompressed transparently

[delner]

This was released in v0.1.4.