feat(v2): stream multipart uploads instead of buffering them

[yagop]

• npm run check is clean (typecheck src + test + examples, lint:core, check:edge, unit suite: 130 pass on Node; on Bun 129 pass + 1 skip proxied, 130 pass direct)
• npm run build is clean
• Generated files regenerated (doc/api.md); generated API sources untouched

Description

Uploads used to hold 2-4x the file size in RAM: fromPath read the whole file, encodeForm copied every part into FormData Blobs, and a ReadableStream InputFile was drained into an in-memory Blob before the send. This PR replaces the FormData encoder with a hand-rolled multipart/form-data body built as a web ReadableStream and passed straight to fetch (duplex: "half"), so file bytes flow from their source without ever being materialized. Measured on a 100MB upload: ~1MB retained vs 200-300MB before.

• src/core/multipart.ts - the streaming encoder. The body is laid out once as ordered pieces (encoded part headers + each file's Blob/Uint8Array/stream/factory); each send attempt streams the pieces into a fresh ReadableStream. Web-standard APIs only; lint:core + check:edge stay green.
• Retry semantics are explicit: Blob/Uint8Array uploads re-stream on retry (byte-identical); a bare ReadableStream is one-shot - sent once, a failure surfaces immediately; the new InputFileStreamFactory (() => ReadableStream | Promise<ReadableStream>) opens a fresh stream per attempt and stays retryable.
• fromPath() returns a disk-backed Blob via fs.openAsBlob (Node >= 19.8, feature-detected; older Nodes fall back to readFile), so disk uploads are flat-memory and retry-safe.
• Runtimes that cannot stream a request body fall back to sending the same bytes as one buffered, replayable Blob, chosen by a once-per-process probe. Bun is excluded from streaming only when an HTTP(S) proxy is configured (read via Bun.env, which Bun honors automatically): its fetch streams request bodies fine on a direct connection, but a stream body routed through a CONNECT proxy stalls forever (fetch() with a ReadableStream request body stalls when routed through an HTTP(S) CONNECT proxy (direct connections work) oven-sh/bun#33918; Node and curl pass through the same proxy, so no local probe can catch it). Direct-connection Bun streams like Node/Deno; drop the guard when the upstream issue is fixed.
• meta.contentType is guarded against CR/LF header injection into part headers; part names/filenames use the WHATWG escaping.
• inputFileToBlob is removed - nothing converts to FormData anymore.
• Default per-request timeoutMs raised 30s -> 5 min so large uploads are not cut off mid-stream (getUpdates already derives its own long-poll timeout).

Validation

• Unit suites on Bun and Node, plus the full local gate.
• Live against api.telegram.org on Node: plain-bytes, fromPath, one-shot stream, stream factory, and nested attach:// media-group uploads (chunked transfer-encoding accepted; confirmed independently with curl).
• Live against api.telegram.org on Bun via the buffered fallback, plus the scoped e2e upload methods.
• Synthetic 100MB upload memory measurement before/after (200-300MB retained -> ~1MB), including a full replay/retry re-send.

Supersedes #1331 (same goal, independently implemented): adopts its InputFileStreamFactory and content-type hardening, and additionally removes the FormData double-copy for Blob/Uint8Array uploads and adds the no-stream fallback that keeps proxied Bun working (streamed uploads there stall through CONNECT proxies, which local-HTTP validation does not catch).

🤖 Generated with Claude Code

[yagop]

The Bun streaming exclusion now has an upstream anchor: filed oven-sh/bun#33918 (oven-sh/bun#33918) with a dependency-free repro - fetch() with a ReadableStream request body stalls forever against HTTPS origins on Bun 1.3.14 (latest stable), while the same bytes as a plain body and the same stream against plain-HTTP origins both succeed; Node 26 passes all four combinations. The supportsRequestStreams guard comment references the issue; when it is fixed upstream, the guard comes off and Bun streams like Node/Deno.

[yagop]

Upstream resolution: Bun triage confirmed oven-sh/bun#33918 and root-caused it to the CONNECT-tunnel stream-body path (fixed by oven-sh/bun#32635, first shipping in Bun 1.4.0; verified here on canary through the same proxy that stalled - all four combinations pass). The guard is now version-gated: only proxied Bun < 1.4.0 takes the buffered fallback, so the workaround retires itself as users upgrade. All four guard branches are unit-verified.