(smile) `SmileGenerator.writeNumber(String)` validates number length against default `StreamReadConstraints` instead of configured ones

\`SmileGenerator._writeIntegralNumber()\` / \`_writeDecimalNumber()\` (used by \`writeNumber(String)\`) guard against too-long number Strings via \`_streamReadConstraints().validateIntegerLength()\` / \`validateFPLength()\`.

But \`_streamReadConstraints()\` returns \`StreamReadConstraints.defaults()\`, so a \`maxNumberLength\` configured on the \`SmileFactory\` is ignored on the write path — both a lower limit (not enforced) and a higher limit (spuriously rejected).

Since \`IOContext\` carries the factory's \`StreamReadConstraints\` (2.15+), the generator can use \`_ioContext.streamReadConstraints()\` instead.

Start in SmileGenerator at _writeIntegralNumber(), _writeDecimalNumber(), and _streamReadConstraints(), then inspect how IOContext carries constraints from SmileFactory. Verify writeNumber(String) with configured lower and higher maxNumberLength values, and add or run regression tests showing that the configured limit, rather than the default, determines acceptance.

Written by the indexing model from the issue text.