Tier 2: validate symbol path segments (reject '..' and '/') to prevent same-host path confusion

[MarketDataApp]

Tier 2 per SECURITY.md — a compatibility-affecting change; needs maintainer approval before merge. Surfaced by the security review behind #39.

Problem

PathSegments.encode (PathSegments.java:24) deliberately preserves / as a separator and URLEncoder leaves . unencoded, so .. passes through intact. buildUri writes the path verbatim and URI.create does not normalize dot-segments (HttpTransport.java:365). Strict single-segment symbols have no character validation.

An application that forwards untrusted end-user ticker input into a symbol path lets that end-user inject extra path segments or dot-segments, e.g. stocks().quote(StockQuoteRequest.of("../../user")) → …/v1/stocks/quotes/../../user/, which a server/proxy may resolve to a different endpoint — served with the application's own token.

Blast radius is bounded: the authority can't be changed (no SSRF), and .. can only strip path segments — so this is same-host, same-credentials path confusion, not remote code/host control. The risk is specifically the untrusted-symbol passthrough pattern.

Proposed fix (needs approval — Tier 2)

Add a strict single-segment encoder for symbol endpoints that rejects (or percent-encodes) / and rejects ./..-only segments, while keeping the current slash-preserving PathSegments.encode only for the free-form options/lookup/{userInput} route (which legitimately matches across slashes). Alternatively validate symbols against an allowed character set at build() time.

Why Tier 2

Tightening symbol validation could reject inputs the SDK currently accepts — a behavior change for existing callers.