CAMEL-23591: camel-mail - align consumer-side dispatch header constant names with Camel naming convention (#23478)

Renames the three consumer-side dispatch header string values in MailConstants
that control post-processing of a consumed mail message (copyTo, moveTo,
delete) to CamelMail<Name>, following the convention used across the rest of
the Camel component catalog and matching the pattern established in CAMEL-23526
(camel-cxf), CAMEL-23522 (camel-mail mail.smtp.* hardening, of which this is
the companion), CAMEL-23461 (camel-aws-bedrock), CAMEL-23532
(camel-vertx-websocket / camel-atmosphere-websocket / camel-iggy), and
CAMEL-23576 (camel-jira).

- MAIL_COPY_TO: "copyTo" -> "CamelMailCopyTo"
- MAIL_MOVE_TO: "moveTo" -> "CamelMailMoveTo"
- MAIL_DELETE: "delete" -> "CamelMailDelete"

The Java field names are unchanged so routes referencing the constants
symbolically continue to work; routes using the literal string values must be
updated (documented in the 4.21 upgrade guide).

The standard RFC 5322 message header constants (MAIL_SUBJECT=Subject,
MAIL_FROM=From, MAIL_TO=To, MAIL_CC=Cc, MAIL_BCC=Bcc, MAIL_REPLY_TO=Reply-To,
MAIL_CONTENT_TYPE=contentType) are intentionally unchanged, as they map
directly to the corresponding email fields and renaming them would break mail
interoperability. The equally-named copyTo and moveTo endpoint URI options on
the mail consumer are also unchanged; only the Exchange header values are
affected.

The generated Endpoint DSL header accessors on MailHeaderNameBuilder have been
renamed: copyTo() -> mailCopyTo(), moveTo() -> mailMoveTo(), and delete() ->
mailDelete().

All existing tests pass (they use symbolic constant references or the
unchanged URI options).

Tracker: CAMEL-23577

Reported by Claude Code on behalf of Andrea Cosentino

Signed-off-by: Andrea Cosentino <ancosen@gmail.com>