common/parser: handle reasoning budget (#20297)

* v1

* Finished!

* Handlie cli

* Reasoning sampler

* Apply suggestions from code review

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>

* Less explosive terminology :)

* Add utf-8 case and tests

* common : migrate reasoning budget sampler to common

* cont : clean up

* cont : expose state and allow passing as initial state

* cont : remove unused imports

* cont : update state machine doc string

---------

Co-authored-by: Sigbjørn Skjæret <sigbjorn.skjaeret@scala.com>
Co-authored-by: Alde Rojas <hello@alde.dev>

common/unicode.h

@@ -20,6 +20,9 @@ struct utf8_parse_result {
 // Returns 0 for invalid first bytes
 size_t common_utf8_sequence_length(unsigned char first_byte);
 
+// Check if a string ends with a complete UTF-8 sequence.
+bool common_utf8_is_complete(const std::string & s);
+
 // Parse a single UTF-8 codepoint from input
 utf8_parse_result common_parse_utf8_codepoint(std::string_view input, size_t offset);