core: enforce bounds checks on input-derived lengths

[TristanInSec]

Add input length validation in several code paths:

• transformations: account for base64 4/3 expansion in b64encode output length check
• parser/parse_body: validate remaining buffer length before delimiter comparison in multipart boundary search
• net/proto_tcp: validate Content-Length value before multiplication to prevent integer wraparound
• sipmsgops: enforce header name length limit in sip_to_json conversion
• msg_translator: validate total URI length in construct_uri before writing components