fix(ra-rpc): sanitize the method name in the request span

[kvinwang]

Follow-up to #1401.

Problem

declare_prpc_routes! records the percent-decoded <method> route segment as the method field of the request span. The span prefixes every event logged while the request runs, so POST /prpc/Info%0A<text> still puts a line starting with <text> into the service log:

...prpc_post{id=45 method=AcmeInfo
DSTACK-FORGED-LOG-LINE-4c2e9b INFO forged}: ra_rpc: dispatching request: AcmeInfo DSTACK-FORGED-LOG-LINE-4c2e9b INFO forged

Fix

Record the field through ra_rpc::log_text, the helper #1401 added for the dispatch and error lines, on both the POST and GET routes. No other caller-controlled span field exists on this path; Rocket's own request span logs the URI still percent-encoded.

Verification

• cargo test -p ra-rpc --all-features: 22 + 4 + 10 + 1 passed
• cargo clippy -p ra-rpc -- -D warnings, cargo fmt --all: clean; cargo check -p dstack-kms -p dstack-vmm -p dstack-guest-agent: ok
• Acceptance suite (PR test: add colocated core component acceptance suite #841) request-contract cases tc-gw-contract-001..003 against a locally built gateway: invariant L7 (no log line begins with caller text after <Method>%0A<marker>) failed on next with 2 forged lines per probe, and passes with this change (0 forged lines, 0 violations across 45/45/626 calls).