wrangler r2 object put: Add --if-none-match flag for conditional uploads

[alexminza]

Describe the solution

Summary

wrangler r2 object put should support conditional uploads via --if-none-match (and other S3-compatible conditional headers), allowing "put-if-not-exists" semantics without a separate check.

Current behavior

wrangler r2 object put always overwrites the target object unconditionally. To avoid overwriting, you must first run wrangler r2 object get to check existence, then conditionally put — introducing a race window and requiring error suppression on the get.

# Current workaround: check-then-put (not atomic)
if ! wrangler r2 object get "bucket/key" --remote --pipe > /dev/null 2>&1; then
  wrangler r2 object put "bucket/key" --remote --file config.json
fi

## Proposed solution
Add `--if-none-match` and `--if-match` flags to `wrangler r2 object put`, mapping to the S3-compatible conditional request headers that R2 already supports:

```bash
# Atomic put-if-not-exists
wrangler r2 object put "bucket/key" --remote --file config.json --if-none-match "*"

# Atomic put-if-unchanged (optimistic concurrency)
wrangler r2 object put "bucket/key" --remote --file config.json --if-match "etag-value"

Exit code should reflect the outcome:

• 0 — upload succeeded
• Non-zero — precondition failed (object already exists / etag mismatch)