fix: sanitize Avro field names on write, respect iceberg-field-name on read

[SreeramGarlapati]

Summary

Iceberg field names can be anything (123column, field.with.dots, etc.) but Avro requires names to match [A-Za-z_][A-Za-z0-9_]*. Java handles this with a sanitize-on-write + restore-on-read protocol using the iceberg-field-name custom Avro property. iceberg-rust was doing neither — writing invalid names directly and ignoring the property on read.

This causes two interop failures:

1. Write path: iceberg-rust produces invalid Avro when field names have leading digits or special chars → strict parsers (Java, Python) reject the file
2. Read path: Avro files written by Java with sanitized names get the wrong field names when read by iceberg-rust

Changes

Write path (Iceberg→Avro conversion in SchemaToAvroSchema::field):

• Added is_valid_avro_name() — checks [A-Za-z_][A-Za-z0-9_]*
• Added sanitize_avro_name() — matches Java's AvroSchemaUtil.sanitize():
  • Leading digit: prefix _ (e.g., 123col → _123col)
  • Special chars: _x<HEX> (e.g., field.name → field_x2Ename)
  • Operates on UTF-16 code units to match Java's charAt() behavior for supplementary chars
• When sanitization is needed: stores original name in iceberg-field-name property

Read path (Avro→Iceberg conversion in AvroSchemaToSchema::record):

• Checks iceberg-field-name custom attribute first, falls back to Avro field name

Java reference

• AvroSchemaUtil.sanitize()
• TypeToSchema.struct() — stores ICEBERG_FIELD_NAME_PROP
• AvroSchemaUtil.ICEBERG_FIELD_NAME_PROP

Closes #2535

Test plan

• test_is_valid_avro_name — validates detection of invalid names
• test_sanitize_avro_name — ASCII edge cases match Java behavior
• test_sanitize_avro_name_unicode — BMP and supplementary char handling (surrogate pairs)
• test_sanitization_round_trip — Iceberg→Avro→Iceberg preserves original names
• test_avro_to_iceberg_uses_iceberg_field_name_property — reads Java-written schemas correctly
• All 1301 existing tests pass (no regressions in bidirectional schema conversion tests)