[backport camel-4.18.x] CAMEL-23591: camel-mail - align consumer-side dispatch header constant names with Camel naming convention

[oscerd]

Backports #23478 to camel-4.18.x.

Cherry-pick of merge commit a04605ea8b8 with one adaptation: the original
PR's upgrade-guide entry was added to camel-4x-upgrade-guide-4_21.adoc,
which does not exist on the 4.18.x maintenance branch. Per the project's
backport policy the same content has been added to
camel-4x-upgrade-guide-4_18.adoc instead (as a new
=== camel-mail - potential breaking change section, separate from and
following the existing === camel-mail section that was added by CAMEL-23522
for the mail.smtp.* JavaMail session-property hardening).

Summary

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>. These sat outside the
Camel namespace and were therefore not filtered by the default
HeaderFilterStrategy.

Constant	Previous value	New value
MailConstants.MAIL_COPY_TO	copyTo	CamelMailCopyTo
MailConstants.MAIL_MOVE_TO	moveTo	CamelMailMoveTo
MailConstants.MAIL_DELETE	delete	CamelMailDelete

Intentionally left unchanged

• RFC 5322 message headers (MAIL_SUBJECT=Subject, MAIL_FROM=From,
  MAIL_TO=To, MAIL_CC=Cc, MAIL_BCC=Bcc, MAIL_REPLY_TO=Reply-To,
  MAIL_CONTENT_TYPE=contentType) map directly to email fields; renaming
  would break mail interoperability.
• The equally-named copyTo / moveTo endpoint URI options on the mail
  consumer are config properties, distinct from the Exchange headers, and are
  unchanged.

DSL accessor renames: copyTo() → mailCopyTo(),
moveTo() → mailMoveTo(), delete() → mailDelete().

Test plan

• mvn test in components/camel-mail on the 4.18.x branch — 218 tests
  pass (4 skipped)
• Cherry-pick applied cleanly except for the 4_21 → 4_18 guide-file
  rename described above
• Upgrade-guide entry added under
  === camel-mail - potential breaking change in
  camel-4x-upgrade-guide-4_18.adoc, distinct from the existing
  CAMEL-23522 === camel-mail section

Tracker: CAMEL-23577

Reported by Claude Code on behalf of Andrea Cosentino

[gnodet]

Clean backport of the camel-mail header rename to 4.18.x — copyTo → CamelMailCopyTo, moveTo → CamelMailMoveTo, delete → CamelMailDelete. Consistent with the ongoing header-naming convention work on main. Upgrade guide entry properly included for the 4.18 line.

LGTM.

Fully automatic review from Claude Code