sdp_ops: implement token-indexed writes for $sdp.line/.stream/.session

[Lt-Flash]

Related to #4135 (companion to #4237, which fixes the crash itself).

$sdp.line(o=/[5]) = "0.0.0.0"; — a token-indexed write — was a silent no-op: the handle_token_edit: label in pv_set_sdp_line(), pv_set_sdp_stream() and pv_set_sdp_session() was an empty stub. The statement located the line, did nothing, set SDP_OPS_FL_DIRTY and returned success. Token indexing existed in the getters but was never implemented in the setters — which is also the answer to the second question in #4135 (o=/[4] = "IP6" "flags dirty without visible changes"). The DIRTY flag alone was enough to re-derive msg->body from sdp_ops' buffer, i.e. the no-op write is what armed the crash fixed in #4237.

Implemented, following docs/manual/Script-CoreVar.md:

• one sdp_ops_find_token() shared by readers and writers, so "token N" cannot mean two different things; the three getters now use it instead of three copies of the token walk;
• one sdp_ops_set_token() behind all three handle_token_edit: labels;
• a token prefix is a key that survives the write: $sdp.line(a=fmtp:96/bitrate=) = "64000" yields bitrate=64000; matched by bare index (o=/[5]) the whole token is the value;
• deleting a token takes the whitespace on one side with it (no double space left); a line left empty is dropped rather than kept blank;
• a value containing an EOL is refused (a token lives inside one line); [insert] / [insertAfter] are refused on tokens, being line operations.

Verified with a 15-assertion script rig (read-side forms from the manual, replace by index, replace by prefix keeping the key, delete without double spaces, untouched lines intact, the $sdp.session path) — run first against the unpatched build, where every edit assertion failed with AFTER == BEFORE. Builds clean with -Wredundant-decls -Wold-style-definition -Wmissing-field-initializers -Werror on current master, independently of #4237.

Two examples in the manual are self-inconsistent and were left alone: m=audio[1]/[0] is documented as audio but the reader has always returned m=audio, and a=nortpproxy:/[0] = "yes" cannot work under whitespace tokenisation.