GH-3317: Fix bytes written by VariantBuilder.appendFloat

[ArnavBalyan]

Rationale for this change

• Fixed the bug by correcting the bytes written value to 4 in VariantBuilder.appendFloat()
• Added test to ensure we only write 5 bytes into the buffer (1 header + 4 float)
• Should prevent any over-indexing the buffer

Are these changes tested?

• Yes

Are there any user-facing changes?

• Yes

Closes: #3317

[ArnavBalyan]

cc @gszadovszky could you pls take a look