fix(encoding): throw on buffer overflow instead of silent truncation in encodeVarint (#7147)

[LeSingh1]

Fixes #7147.

encodeVarint walked the loop with i <= Math.min(buf.length, MaxVarintLen64). For a value that needed all 10 uint64 varint bytes plus a continuation, the 11th iteration wrote to buf[10] on the default 10-byte buffer — silently dropped by Uint8Array — and the function returned [Uint8Array(10), 11], a slice shorter than the byte count it reported. Callers then fed that truncated buffer to decodeVarint and got a wrong number back. Repro from the issue:

console.log(encodeVarint(0x1234567891234567891n));
// before: [Uint8Array(10), 11]   (looks fine, but the slice is missing a byte)
// after:  RangeError: Cannot encode the input ... into varint as it overflows uint64

Fix

• Cap the loop at min(buf.length - offset, MaxVarintLen64), write through offset + i so the offset path can't drift either.
• Split the post-loop throw into two messages: "buffer holds at most N byte(s) after offset" when the caller (or default) supplied a short buffer, and the existing "overflows uint64" when the value genuinely exceeds the protocol limit. Both are RangeError so existing assertThrows(_, RangeError) callsites still pass.
• The happy path is byte-for-byte identical: same returned slice, same byte count.

Tests

Three cases added to encoding/varint_test.ts:

• throws when the default buffer is too small (#7147) — exact repro from issue, asserts RangeError with "overflows uint64"
• throws when a caller-provided buffer is too small — 200000 (3 varint bytes) into a 2-byte buffer
• throws when offset leaves no room in the buffer — 10-byte buffer with offset=10

$ deno test --allow-read encoding/varint_test.ts
ok | 18 passed | 0 failed (5ms)

$ deno lint encoding/varint.ts encoding/varint_test.ts && deno check encoding/varint.ts encoding/varint_test.ts
Checked 2 files
Check encoding/varint.ts
Check encoding/varint_test.ts

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (f0c9f14) to head (71170f6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7159   +/-   ##
=======================================
  Coverage   94.57%   94.57%           
=======================================
  Files         636      636           
  Lines       52138    52141    +3     
  Branches     9399     9400    +1     
=======================================
+ Hits        49311    49314    +3     
  Misses       2249     2249           
  Partials      578      578           

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