Bug: JSON_IETF path key stripping

[steiler]

Summary

stripPrefixFromValue in sdcpb/path_additions.go strips module prefixes from path key values
by blindly splitting on the first ":" in each slash-separated segment. This is unsafe because
not every ":" in a value is a YANG module separator — IPv6 addresses and other colon-containing
strings are valid key values in many YANG models and must not be modified.

Why this is an issue

In RFC 7951 (JSON_IETF), module-qualified names only appear in JSON object keys and
identityref values. Generic string key values (e.g. inet:ip-address, inet:ipv6-address)
are not module-qualified — they are plain strings like 2001:db8::1. Any code that applies
: splitting unconditionally to path key values will silently corrupt those addresses.

Concrete example

A YANG list keyed by inet:ipv6-address:
/neighbor[address=2001:db8::1]/admin-state

stripPrefixFromValue("2001:db8::1") sees a : and cuts on it, producing db8::1.
The resulting path no longer matches the stored entry, silently breaking lookups.

Code occurrence

• sdcpb/path_additions.go — stripPrefixFromValue (called from StripPathElemPrefixPath):
  strips the first : from every slash-separated segment of a key value without checking
  whether the segment is actually a module-qualified identifier.

Note on JSON object key stripping

The separate colon splitting on JSON object keys (e.g. srl_nokia-interfaces:interface → interface)
is intentional and correct: RFC 7951 object keys are always [module:]name identifiers, never
IPv6 addresses or other ambiguous strings. That code is not affected by this issue.

Expected behavior

Only strip a module prefix from a key value segment when it is unambiguously a YANG-qualified
identifier. At minimum: do not strip segments that contain more than one :.

Suggested fix direction

• Short term: guard stripPrefixFromValue to only strip segments with exactly one :,
  leaving multi-colon segments (IPv6, etc.) intact.
• Long term: make the stripping schema-aware and apply it only for identityref-typed leaves.

Acceptance criteria

• Regression test: IPv6 key value 2001:db8::1 is preserved after StripPathElemPrefixPath.
• Regression test: mixed slash-separated value mod:a/2001:db8::1/mod:c strips only the
  unambiguous segments, yielding a/2001:db8::1/c.
• Existing behaviour for module-qualified identityref values (single :) is unchanged.