rpc: Use self.Arg<_>(name) helper over manual and duplicate parsing #36384

The Arg() helper has many benefits:

• It does not require manual isNull checks
• It does not require hard-coding the fallback/default value a second time
• It does not require manually specifying the getter for the type. Specifying the type is enough.

So use it more broadly.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36384
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36384.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36384#author-ralph

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #36566 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36566.svg">} (private broadcast: wait synchronously for mempool acceptance by andrewtoth)
• #36264 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36264.svg">} (wallet rpc: fix stale argument metadata and help text by MrHodlX)
• #36167 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/36167.svg">} ([RFC] Enable -Wunused by fanquake)
• #35511 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35511.svg">} (RFC: consensus: Make CAmount a class by hodlinator)
• #35370 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35370.svg">} (rpc: add key-origin modes to PSBT processing RPCs by junbyjun1238)
• #35302 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/35302.svg">} (Silent Payments: Sending (take 2) by Eunovo)
• #33112 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/33112.svg">} (wallet: relax external_signer flag constraints by Sjors)
• #32857 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/32857.svg">} (wallet: allow skipping script paths by Sjors)
• #32468 _{<img src="https://drahtbot.space/ack_count/bitcoin/bitcoin/32468.svg">} (rpc: generateblock to allow multiple outputs by polespinasa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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🚧 At least one of the CI tasks failed. _{Task iwyu: https://github.com/bitcoin/bitcoin/actions/runs/36623882963/job/109595800670} _{LLM reason (✨ experimental): CI failed because IWYU detected incorrect/missing includes and triggered a “Failure generated from IWYU” (returned exit code 1).}

<details><summary>Hints</summary>

Try to run the tests locally, according to the documentation. However, a CI failure may still happen due to a number of reasons, for example:

• Possibly due to a silent merge conflict (the changes in this pull request being incompatible with the current code in the target branch). If so, make sure to rebase on the latest commit of the target branch.

• A sanitizer issue, which can only be found by compiling with the sanitizer and running the affected test.

• An intermittent issue.

Leave a comment here, if you need help tracking down a confusing failure.

</details>

Concept ACK

Forgot to say that everything here is a refactor, except for the cleanup commit that changes the schema of one RPC:

diff --git a/getopenrpcinfo.full.json b/getopenrpcinfo.full.json
index e0cebef..cbbf785 100644
--- a/getopenrpcinfo.full.json
+++ b/getopenrpcinfo.full.json
@@ -15035,5 +15035,5 @@
                         "type": "number",
-                        "x-bitcoin-default-hint": "3, range=0-4"
+                        "default": 3
                     },
-                    "description": "How thorough the block verification is:\n- level 0 reads the blocks from disk\n- level 1 verifies block validity\n- level 2 verifies undo data\n- level 3 checks disconnection of tip blocks\n- level 4 tries to reconnect the blocks\n- each level includes the checks of the previous levels"
+                    "description": "How thorough the block verification is (range 0-4):\n- level 0 reads the blocks from disk\n- level 1 verifies block validity\n- level 2 verifies undo data\n- level 3 checks disconnection of tip blocks\n- level 4 tries to reconnect the blocks\n- each level includes the checks of the previous levels"
                 },
@@ -15044,5 +15044,5 @@
                         "type": "number",
-                        "x-bitcoin-default-hint": "6, 0=all"
+                        "default": 6
                     },
-                    "description": "The number of blocks to check."
+                    "description": "The number of blocks to check (0=all)."
                 }
diff --git a/verifychain b/verifychain
index b742f17..1863a80 100644
--- a/verifychain
+++ b/verifychain
@@ -5,3 +5,3 @@ Verifies blockchain database.
 Arguments:
-1. checklevel    (numeric, optional, default=3, range=0-4) How thorough the block verification is:
+1. checklevel    (numeric, optional, default=3) How thorough the block verification is (range 0-4):
                  - level 0 reads the blocks from disk
@@ -12,3 +12,3 @@ Arguments:
                  - each level includes the checks of the previous levels
-2. nblocks       (numeric, optional, default=6, 0=all) The number of blocks to check.
+2. nblocks       (numeric, optional, default=6) The number of blocks to check (0=all).