fix(ext/node): convert Buffer string write arguments once

[jackspirou]

Summary

buf.asciiWrite(), buf.latin1Write(), and buf.utf8Write() compared the raw offset and length values and then used them again. Thus a valueOf() method ran more than once. Node.js 24.21 and 26.8 convert each argument once (nodejs/node#65043). The test-buffer-write.js fixture of those releases asserts this.

The methods also skipped the native steps of Node's StringWrite(). These bugs occurred on all Node.js versions:

Call on a 4-byte buffer	Before	Node.js and this PR
utf8Write('abcdef', 0, 0)	3 bytes	0
buf.write('abc', 0, 0)	3 bytes	0
asciiWrite('abcdef', NaN)	3 bytes	0
utf8Write('abcdef', '1', '2')	3 bytes	2

Change

• A shared stringWrite() helper converts each argument once. It truncates each value and clamps the length, as the native write does. It reads the byte length again after conversion, because user code can resize or detach the buffer.
• The bounds checks differ between Node.js releases. A new BufferStringWriteBoundsPolicy lets an embedder select the contract:
  • TruncateThenClamp: Node.js 20 and 22.0 through 22.8. A negative value gives ERR_OUT_OF_RANGE, and a long length is clamped.
  • CheckThenClamp: Node.js 22.9 and later 22.x (buffer: optimize writing short strings nodejs/node#54310, buffer: truncate instead of throw when writing beyond buffer nodejs/node#54524). A long length is clamped.
  • CheckThenReject: Node.js 23 and later (buffer: throw when writing beyond buffer nodejs/node#54588). A long length gives ERR_BUFFER_OUT_OF_BOUNDS. This is the default in worker.rs, web_worker.rs, and snapshot_info.rs.
• hexIndexOutOfRange() is now indexOutOfRange(), because the write path uses it too.

Verification

• unit_node buffer_test: pass. The two new tests pass. Their expected values match Node.js 26.7, except for the single valueOf() call and the BigInt case, which are the #65043 contract.
• node_compat test-buffer-write: 6 of 6 pass.
• cargo test -p deno_node --lib policy: 6 of 6 pass, including the new policy test.
• The CI clippy command, cargo fmt --check, dprint, and tools/lint.js --js: clean.
• The Node 24.21 and Node 26.10 test-buffer-write.js fixtures from Nimbus: all assertions pass.
• Pre-PR autoreview: clean.