rpcclient: add Client.URL() to expose configured RPC server URL

[Dione-b]

Motivation

rpcclient.Client stores the RPC server URL internally (unexported url field) but never exposes it. This forces consumers that wrap Client in another type — such as stellar/friendbot — to duplicate the URL manually, since there's no way to retrieve it from the client. This is redundant and prone to drift between the duplicated URL and the one actually used by the client. For comparison, horizonclient.Client already exposes this via a public HorizonURL field.

Resolves #5885

Behavior

Added the Client.URL() string method (clients/rpcclient/main.go), which returns the RPC URL the client was configured with.

• Implemented as a getter (return c.url) instead of exporting the url field directly — preserves encapsulation and allows future flexibility without breaking the public API (the alternative suggested and discarded in the issue itself).
• c.url is written only in NewClient and never mutated afterward (including during refreshClient, which only recreates the internal jrpc2 connection) — so the getter needs no lock (c.mx) and is safe for concurrent use.
• Package documentation (clients/rpcclient/doc.go) updated to mention Client.URL and its use case (client wrapped in another type).

There is no internal usage in the SDK — this is a public API for external consumers, analogous to horizonclient's HorizonURL.

Tests

Two new tests in clients/rpcclient/main_test.go:

• TestClient_URL: verifies URL() returns exactly the URL passed to NewClient.
• TestClient_URL_afterRefresh: triggers a GetHealth call against a server that returns invalid JSON, forcing callResult to internally trigger refreshClient() (see main.go:75-78), and confirms URL() still returns the same URL after that refresh cycle.

Validation performed:

• go vet ./... — no warnings
• go test -race ./... (rpcclient package) — all tests pass, including the two new ones, under the race detector
• gofmt -l . — no diffs

Release Impact

No breaking changes. Pure addition of a public API (Client.URL()), with no change to existing method signatures or behavior. No data migration required. Entry added to CHANGELOG.md under ## Pending / ### New Features.

Checklist

• Public and internal documentation updated to reflect behavior changes.
• Error messages and failure codes reviewed or updated to cover new scenarios.
• Command examples and tutorials use isolated data or local test environments, adhering to security best practices.
• Dependencies, lockfiles, and CI/CD workflows are synchronized with the new changes.
• All automated tests and linting checks pass successfully.

[Copilot]

Pull request overview

Adds a public getter for retrieving an RPC client’s configured server URL.

Changes:

• Adds Client.URL().
• Adds unit tests and an executable example.
• Updates package documentation and changelog.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
clients/rpcclient/main.go	Implements the URL getter.
clients/rpcclient/main_test.go	Tests URL retrieval and refresh behavior.
clients/rpcclient/examples_test.go	Demonstrates URL retrieval.
clients/rpcclient/doc.go	Documents the new API.
CHANGELOG.md	Records the new feature.

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[Dione-b]

@leighmcculloch friendly ping — rebased on latest main, CHANGELOG conflict resolved, CI green. Would appreciate a review when you have a moment.

[leighmcculloch]

Thank you!