Add CAP-0073 ingestion support and bump to protocol 26 - #171

The P28 integration leg reads testdata/unlimited-config-v28.xdr in
upgradeLimits() (internal/test/integration/integration.go:787); it was missed
in the P28 bump, so every P28 test going through StartHorizon setup failed with
"open testdata/unlimited-config-v28.xdr: no such file or directory". CAP-0083
adds no Soroban config settings, so the v28 ConfigUpgradeSet is byte-identical
to v27 under the P28 go-stellar-sdk pin (v26 and v27 are already byte-identical).

The protocol matrix is [27, 28], so v26 is now unreferenced; drop it to keep
testdata aligned with the matrix, matching the rolling-window precedent from
stellar#171/stellar#189.

* Re-pin go-stellar-sdk to the CAP-0083 XDR regeneration and bump
MaxSupportedProtocolVersion to 28.
* Add v28 load-test fixtures
* CI: run integration tests against Protocol 28

Switch the integration matrix to protocol-version [27, 28]. P28 uses the
vnext (next-protocol-enabled) core build 27.0.1-3348.ff61f2e6d. The P28
stellar-rpc image is left as a commented placeholder until stellar-rpc#789
publishes one, so that leg stays amber rather than testing against a P27 RPC.
Repoint the cache source hash to the P28 (latest) + P27 (previous) images.

* CI: build a P28 stellar-rpc image from stellar-rpc#789 instead of failing the leg

The P28 matrix leg had no PROTOCOL_28_STELLAR_RPC_DOCKER_IMG (no published
P28-capable stellar-rpc image exists; stellar-rpc#789 is still a draft over
protocol-next and the repo only publishes images from main/release/**). The
"Pull and set Stellar RPC image" step ran `docker pull ""` -> invalid
reference format -> the P28 legs went red and fail-fast cancelled the P27
legs. Leaving the image empty was not a safe "amber": the compose default
(stellar/stellar-rpc) is a P27 RPC, so the soroban suites would have run
against the wrong RPC.

- Guard the RPC pull step so a missing prebuilt image is a no-op.
- Build a genuine P28 stellar-rpc image in-CI from stellar-rpc#789 source,
  fetched by pinned SHA via refs/pull/789/head (there is no p28-cap-0083
  branch on stellar/stellar-rpc; the head lives on a fork), plus the pinned
  P28 captive-core deb, and run all six EnableStellarRPC suites (sac,
  invokehostfunction, extend_footprint_ttl, txsub, txsub_async, transaction)
  against it on the P28 leg.
- If that build is unavailable, skip those suites loudly (tracked to
  stellar-rpc#789) rather than silently falling back to a P27 RPC.

* CI: drop P28 stellar-rpc from-source build; reuse the P27 RPC image

CAP-0083 is consensus-layer (skip ledgers); the RPC layer is unaffected,
so the P27 stellar-rpc image is functionally adequate for the P28
integration leg. Reverts the from-source-build pipeline added in a9fe4d7
in favor of the simple two-line "use previous protocol's image + TODO".

Drops:
- PROTOCOL_28_STELLAR_RPC_PR_SHA env var
- "Build P28 stellar-rpc image from stellar-rpc#789" step
- empty-IMG handling in "Pull and set Stellar RPC image"
- PR_SHA reference in the source-hash key

Restores the original "Pull and set Stellar RPC image" step. When a
real P28 stellar-rpc image is published, replace the
PROTOCOL_28_STELLAR_RPC_DOCKER_IMG value with the new tag and drop
the TODO.

* Roll integration test fixture window to {v27, v28}

The P28 integration leg reads testdata/unlimited-config-v28.xdr in
upgradeLimits() (internal/test/integration/integration.go:787); it was missed
in the P28 bump, so every P28 test going through StartHorizon setup failed with
"open testdata/unlimited-config-v28.xdr: no such file or directory". CAP-0083
adds no Soroban config settings, so the v28 ConfigUpgradeSet is byte-identical
to v27 under the P28 go-stellar-sdk pin (v26 and v27 are already byte-identical).

The protocol matrix is [27, 28], so v26 is now unreferenced; drop it to keep
testdata aligned with the matrix, matching the rolling-window precedent from
#171/#189.