fix: Drop misleading `const` on TLV-populated descriptor fields

safe_descriptor_t::address and four gating_t fields (hash_selector,
intro_msg, tiny_url, address) were declared `const` to document that
they should not change after parsing, but the TLV handlers wrote
into them through `(uint8_t *)` / `(char *)` casts that strip the
qualifier. The underlying storage comes from APP_MEM_CALLOC() so
this is not strict undefined behaviour, but it is a pattern several
static analyzers flag and it weakens the type signal at every read
site.

Remove the `const` from the five fields and drop the corresponding
casts at the parser call sites where they were stripping the
qualifier. The two remaining `(uint8_t *)` casts on `char` fields
stay since they still perform a char->uint8_t conversion required by
the TLV helper signatures.

(cherry picked from commit 5a4f7f24b540ee7b84203cf8e2481a5d9b685f0a)